The effect of platelet activating factor (PAF) on three mechanical [maximal mechanical tension (Pmax); time to peak tension; maximal rate of rise of tension (+dP/dt)] and four electrical [action potential duration (APD); resting membrane potential; overshoot; maximum rate of depolarization] parameters of cardiac function was studied on fragments of isolated human cardiac papillary muscle. 20 specimens of small tissue fragments excised from the left ventricle by open heart surgery were challenged with various doses of synthetic PAF (10––10––10––6M). PAF, but not its biologically inactive 2-lyso-derivative (lyso-PAF), induced a biphasic dose-dependent effect, characterized by a transient positive effect on inotropism (increased Pmax, + dP/dt) and of APD, followed by a marked, prolonged negative effect on both inotropism (decreased Pmax, time to peak tension, + dP/dt) and APD. No changes in resting membrane potential, overshoot and maximum rate of depolarization were detected after PAF challenge. Propranolol (2 × 10––7M) completely prevented the positive inotropic effect suggesting a stimulation of β-receptors, possibly exerted by endogenous catecholamines. In-domethacin (1 × 10––4M) did not modify the initial positive effect, but markedly reduced the subsequent negative effect induced by PAF on inotropism. These findings are consistent with the interpretation that the effect of PAF on the inotropism is related to liberation of cyclooxygenase-derived metabolites.