Chronic stimulation of a predominantly fast skeletal muscle enhanced the expression of type I (slow muscle) Ca-ATPase and suppressed the expression of the type II (fast muscle) Ca-ATPase. Monoclonal antibodies IID8 and IIH11 against type I (slow) and type II (fast) isozymes respectively, were used to type the Ca-ATPases of the isolated SR (sarcoplasmic reticulum) by Western blots, and the Ca-ATPases of the muscle fibers by immunohistochemistry. Of the fibers from control muscles 80% stained for the type II isozyme and 20% for the type I isozyme. Following chronic stimulation all fibers stained for type I isozyme and none stained for type II isozyme. Ca-ATPase isozyme distribution in isolated SR confirmed this effect of chronic stimulation. The calcium uptake activities of homogenates of stimulated muscles were 22% of the control muscles. The Ca-ATPase and calcium-uptake activities of the isolated SR from stimulated muscles were, respectively, 32 and 45% of the control muscles.