A method for the quantitative study of the actomyosin ATPase activity (Ca,MG-ATPase) in thin sections cut in a cryostat is presented. This method is based on the liberation of 32P from [gamma 32P]ATP or 45Ca phosphate precipitation. The advantage of this method lies in the requirement for only a small muscle sample and the availability of serially cut sections for other assays including Ca uptake by sarcoplasmic retciulum and histochemical tests for oxidative and glycolytic enzymes. The actomyosin ATPase activity for various types of muscles determined by this method showed the same sequence found in isolated protein, that is, fast-twitch skeletal greater than slow-twitch skeletal greater than cardiac. The Ca,Mg-ATPase of cryostat sections showed Ca sensitivity. The fact that the sections retained Ca sensitivity at 37 degrees C, in contrast to myofibrils, which have been reported to lose Ca sensitivity at this temperature, indicates that the structural integrity of the contractile and regulatory apparatus is preserved to a higher degree in sections than in isolated myofibrils.