The energy metabolism of exogenous and endogenous substrates was investigated in Ca-tolerant adult ventricular myocytes in short-term culture. CO2 production from exogenous glucose (5 mM), lactate (5 mM), and palmitate (100 microM) were 8, 20, and 29 mumol X h-1 X g wet wt-1. Endogenous lipolysis shared in energy production by 78, 61, and 31%, respectively. Fatty acids represented the main respiratory fuel even when 10(-7) M insulin, 5 mM lactate, or 5 mM dichloroacetate were supplied in addition to glucose. With palmitate, triglyceride contents were doubled within 3 h of cell incubation. With 5 mM glucose as sole exogenous substrate the energy turnover in these resting cells resembled that of arrested hearts, corresponding to 2 ml O2 X min-1 X 100 g wet wt-1. It is concluded that the basic features of substrate metabolism resemble those of the intact myocardium in that fatty acids represent the major fuel, but the contribution of endogenous lipolysis is greater than in the beating heart.