The positive inotropic responses to isoprenaline, dobutamine, histamine, forskolin, isobutylmethylxanthine (IBMX), dibutyryl‐cyclic adenosine monophosphate (db‐cAMP), ouabain and calcium were studied in isolated, electrically driven papillary muscle strips from either terminally failing human hearts or non‐failing donor myocardium. The positive inotropic effect of calcium has been taken to evaluate the maximal force of contraction of each individual muscle strip (‘contractile reserve’). In the non‐failing heart, the maximal positive inotropic effect of isoprenaline, dobutamine, IBMX, ouabain and calcium were not significantly different, but were significantly greater than histamine. In terminally failing hearts, the positive inotropic effects of agents stimulating the adenylate cyclase by a receptor‐dependent mechanism (isoprenaline, dobutamine and histamine) and the phosphodiesterase inhibitor IBMX are less than in the normal heart. Furthermore, these compounds gave a markedly reduced inotropic effect compared with forskolin, db‐cAMP and ouabain, which gave maximal responses similar to calcium in the failing hearts. The data did not differ when the increase of force of contraction was related to the diameter of each preparation. These results indicate that a defect in adenylate cyclase occurs in the failing human heart, presumably located at the regulatory stimulatory subunit (G_s) of the adenylate cyclase since effects through stimulatory receptors were reduced. Responses from activation of the catalytic subunit or through cAMP‐dependent protein kinases were less affected. Since the positive inotropic effect of IBMX is also impaired, it is suggested that the basal rate of cAMP production is also reduced in heart failure. Therefore, compounds directly activating the catalytic subunit of the adenylate cyclase or cAMP‐dependent protein kinases or the (Na⁺, K⁺)‐ATPase should be effective positive inotropic agents even in severe heart failure.