When the Allman Brothers Band first pioneered group improvisation in 1968, its flexible rhythms and sustained guitar invention were rare phenomena in rock-and-roll. What's surprising is that more than 20 years later such improvisation in rock-and-roll is still rare. Certainly, few groups are capable of the high levels of musicianship and imagination that the reunited Allman Brothers Band displayed at the Merriweather Post Pavilion Saturday night.

Whether it was a new song like "Low Down Dirty Mean" or an old song like "Whipping Post," the Georgia septet opened the material up with solos that expanded the possibilities of the melody without ever abandoning it. The band's four living founding members were joined by three newcomers, and the new lineup sounded far more comfortable and integrated than it did on its inaugural tour last summer -- or even on its recent album. Gregg Allman took all but four of the lead vocals, and he was in great voice, with a growl that took the group back to its blues roots.

Newcomers Warren Haynes on guitar and Johnny Neel on piano had a tendency to overplay and to get hung up on repetitive, high-pitched phrases, but they also proved gifted and appropriate foils for guitarist Dickey Betts. It was Betts who led Haynes through the sparkling twin-guitar parts and the soaring improvisations that lit up numbers like "Rambling Man" and "Jessica" (which segued into "Mountain Jam"). Betts's newest instrumental, "True Gravity," has evolved since the album version into an epic analogous to his "(In Memory of) Elizabeth Reed." Drummers Jai Jaimoe and Butch Trucks's rippling, pulsing rhythms gave the soloists room to maneuver without losing the groove.

Meat Loaf opened the show with yet another pointless comeback attempt.