At the Merriweather Post Pavilion last night, Joni Mitchell illuminated the labyrinth of the heart that she's been so fervently exploring over the course of her career.

She once defined her task as "a search for love and music," and as she's gone deeper into the one, she's expanded the parameters of the other. Her lyrics mix the confessional and the expose' in equal measure, her wordscapes as vivid as her watercolors, with gorgeous, liquid lines dripping into one another.

Her music is bounded by experimentation, and in the course of a two-hour concert Mitchell moved from elfin dulcimer on "A Case of You" to somber piano on "For Free" to acoustic and (mostly) electric guitar on rollicking rockers like "Raised on Robbery" and "You're So Square," and ended with fluid jazz exercises like "God Must Be a Boogie Man." As always, she was backed by a top-notch quartet.

Mitchell insists on rocking hard at times and ends up strident--but then again, she has always marched to the beat of a different strummer. Mitchell's voice was strong and crystalline, but often descended into the huskier shadings she has developed in recent years. Her sophisticated phrasing was inspired but at times close enough to improvisation to prove difficult to follow.

Mitchell started the evening alone, with "Coyote," and she ended it alone, with a shimmering and sensual "Woodstock" that was half celebration of the past and half encouragement for a future.