Primus drummer Tim Alexander suffered a heart attack Thursday ... and now he's waiting for doctors to fix his ticker via open heart surgery.
Alexander was rushed to a local hospital after he was hit with the attack ... according to a rep for his other band Puscifer.
Tim was the original drummer for Primus ... but he took a 7 year break from the band in '96. Their biggest song was the '93 classic "My Name is Mud."
We're told doctors hope to get him into surgery Monday.