The inotropic effects of caffeine (1-3 mM) were studied in the presence and absence of strophanthidin in canine Purkinje fibers perfused in vitro. Caffeine (1 mM) induced a similar initial increase in contractile force in different calcium solutions (+22, +23 and +24% in 0.54, 2.7 and 8.1 mM calcium, respectively) and when propranolol (3.4 X 10(-6) M) was present. Also, caffeine increased contractile force in high potassium (16.2 mM) at a time when the slow action potentials were unaltered. After the increase, 1 mM caffeine decreased force by about 50%, and the decrease was larger when caffeine (3 mM) or [Ca]0 (8.1 mM) was higher. In the presence of caffeine, strophanthidin (3 X 10(-7)-1 X 10(-6) M) increased force (+302%) if caffeine (0.3 mM) and Ca concentrations (0.54 mM) were low. If either caffeine or calcium was increased, strophanthidin had no effect or decreased force. Strophanthidin alone increased force and then decreased it; caffeine increased force in the first stage and decreased it during the second stage. The positive inotropic effect (+224%) of low sodium (78.4 mM versus 149.4 mM in Tyrode solution) was also abolished by caffeine (-24%). In ventricular muscle fibers, caffeine increased force more (+59%) and reduced force less in the presence of strophanthidin. The results indicate that caffeine increases force initially by releasing calcium from intracellular stores. The caffeine-induced decline in force is modulated by calcium in that it is exaggerated by agents or procedures which increase cellular calcium (strophanthidin, high calcium, low sodium solutions) and is reversed in a low calcium solution.