Metallica, one of the biggest acts yet to make its music available for sale via Apple's iTunes Music Store, has ended its holdout. Beginning yesterday (July 25), the enduring hard rock outfit's work is for sale via the U.S. and Canada editions of the digital store.

Metallica albums have previously been available for download via services such as MSN Music, but have never been broken up for single-song purchase until now. At iTunes, the group's first four albums ("Kill 'Em All," "Ride the Lightning," "Master of Puppets" and "...And Justice for All") have been expanded with previously unreleased live tracks recorded in Seattle in 1989.

"We chose these four because, unlike the more recent releases, we were only capable of writing 8-9 songs for each of these albums," the band writes on its Web site.

The iTunes rollout only includes the U.S. and Canada for now, due to a dispute with Metallica's overseas record label, Mercury. The company "doesn't seem to want to play ball with us on this at the moment," the band says.

Among the other major artists who have yet to come to an agreement with iTunes are the Beatles, Led Zeppelin, Radiohead and Garth Brooks.

In other Metallica news, the group will appear in animated form on the season premiere of "The Simpsons," which will air Sept. 10 on Fox.