After listening to Joni Mitchell for six months straight, Bryden MacDonald longed for something simple. Like maybe Britney Spears.

"I couldn't listen to anything but Joni," laughs the playwright and director of When All The Slaves Are Free, the new theatre production based on the words and music of Mitchell. It opens at the Great Canadian Theatre Company Thursday, after preview performances tonight and tomorrow. "Then I found that just for a break I was thrilled to hear Britney Spears. That was kind of freaking me out. I just needed to hear something with a constant beat."

Evidently it's possible to overload on Mitchell's angular chords and syncopated rhythms. But MacDonald also developed a new appreciation for the sheer talent of the Alberta-born songstress, who's considered by many to be the most influential female recording artist of the 20th century. Her early material, in particular, was a surprise.

"Her first two albums had songs that I did not know," MacDonald says during a smoke break from rehearsals last week, the traffic of Gladstone Avenue buzzing by. "There were some beautiful, amazing songs, and you realize that she was 22 or so when she wrote this stuff. They're magnificent, and musically what she was doing is mind-blowing.

"That's not to say that she didn't grow as a musician or as a composer, because she did, of course, but the starting place, where she began, was so advanced."

He was even more impressed when he found that Mitchell battled polio as a child. Her first performances were singing to the other patients in a children's hospital. She later taught herself to play guitar.

It's not the first time the 41-year-old playwright has immersed himself in a Canadian artist's music for the purpose of bringing it to the stage.

Twelve years ago, he wrote Sincerely, A Friend, a musical play based on the work of Leonard Cohen. While it drew mixed reviews from the critics, Cohen responded with a note to MacDonald promising a box of chocolates and a rose -- which never materialized.

Three years ago, MacDonald tackled Carole Pope and her Juno-winning, gender-bending pop group, Rough Trade, in the production Shaking The Foundations. The anti-diva herself showed up opening night, and apparently loved it.

So far there's no word on how Mitchell feels about her work being subjected to the same treatment, but MacDonald suspects she must know about it.

It took about a year to secure the rights to Mitchell's songs, and even then there were probing questions about how the material would be used.

MacDonald says they wanted to know details such as whether there would be any dancing, if there was other dialogue, and if someone would be playing Mitchell. "It didn't sound like the questions an agent would ask," says MacDonald. "My feeling is that she wants to know what's going on."

Mitchell can rest assured that her songs are in experienced hands. MacDonald has established a national reputation as a playwright with works such as Whale Riding Weather (playing next April at the GCTC), The Weekend Healer and The Exstasy of Bedridden Riding Hood. He is known for the strength of his characters.

When All The Slaves Are Free stars three female singers -- Elizabeth Beeler, Susan Henley and Mary Kelly. Together, MacDonald says, they create one character, although it's not necessarily Mitchell.

Backed by a three-piece jazz combo from Montreal, the women sing 27 of Mitchell's songs, loosely divided between Now, Hindsight and the Journey.

The storyline is "subtextual," MacDonald notes. "There is a kind of path that each of the singers take, but it is pretty non-linear, very heady and surreal -- as the music is."

Growing up in Cape Breton, MacDonald liked tunes that were a lot more in your face. Punk was an early passion -- if he'd had anything more than a passing fancy for Mitchell, he never would have admitted it.

"I loved the Sex Pistols," he says. "I did the whole thing -- safety pins in my ears. It was hard being a punk in Cape Breton. I was the only one."

Theatre and writing were MacDonald's other loves. He acted -- "I wanted to be Rob Lowe" -- and spent his evenings writing in a journal, sometimes for hours at a time.

In high school, he found there weren't many roles for "angry young teenagers," so he wrote his own material.

As a young actor in Toronto, MacDonald struggled. It didn't help, he says, that he wasn't very good at grooming, and kept showing up late to auditions.

Soon he was getting more attention for his writing and directing than acting.

Some 16 years later, MacDonald is ready to get back on stage as an actor. He's been living in Montreal for two years, and feels he's settled down enough not only to shave regularly, but also to keep appointments.

He directs at McGill University, teaches playwrighting at the National Theatre School and is working on a new play.

But combining music and theatre is a "lovely diversion" for the former saxophone player.

"It's not as lonely as writing," he says. "I love this. I sit in a room and three gorgeous voices sing at me all day. It could be worse."

When All The Slaves Are Free runs at the GCTC, 910 Gladstone Ave., from Thursday to Sept. 29. Showtime is 8 p.m. Tues.-Sat., with 2 p.m. matinees Sat. and Sun. Tickets are $26 for evening shows, and $14 for matinees and preview performances tonight and tomorrow. Call 236-5192 for tickets.