In a previous study we described the inhibitory action of a cytosolic protein fraction from heart muscle on ATP-dependent Ca2+ uptake by sarcoplasmic reticulum; further, this inhibition was shown to be blocked by an inhibitor antagonist, also derived from the cytosol (Narayanan et al. Biochim Biophys Acta 735: 53-66, 1983). The present study examined the effects of the endogenous cytosolic Ca2+ transport inhibitor and its antagonist on ATP-dependent Ca2+ uptake by sarcolemmal vesicles isolated from rat and canine heart. The cytosolic inhibitor caused strong inhibition (up to 97%) of Ca2+ uptake by sarcolemma (SL); this inhibition could be reversed by the cytosolic inhibitor antagonist. Studies on the characteristics of inhibition revealed the following: a) Inhibition was dependent on the concentration of the inhibitor (50% inhibition with approximately 80 micrograms inhibitor protein). b) The inhibitor reduced the velocity of Ca2+ uptake without appreciably influencing the apparent affinity of the transport system for Ca2+ but caused greater than 2-fold decrease in its apparent affinity for ATP. c) The rates of unidirectional passive Ca2+ release from actively Ca2+ loaded SL vesicles were not altered by low concentrations of the inhibitor (less than 100 micrograms/ml) which were effective in producing marked inhibition of Ca2+ uptake; at higher concentrations (greater than 100 micrograms/ml), the inhibitor caused increase in the rates of passive Ca2+ release. These findings demonstrate that the activity of the ATP-driven Ca2+ pump of cardiac SL can be regulated in vitro by endogenous cytosolic proteins.