Gossypol, an effective male antifertility agent, had a minor but significant negative inotropic action in isolated ferret right ventricular papillary muscles at concentrations of 1-5 μg/ml. Reduction of peak developed force and maximum extent of shortening was readily demonstrable at three different resting control contractile states achieved through increases in extracellular calcium from 0.625 to 2.5 mM/l. Gossypol had no significant effect on maximum rate of tension rise nor on maximum velocity of shortening. These results suggest that gossypol reduces the toal number of crossbridges without significantly influencing the rate of making and breaking crossbridges and that its negative inotropic action is probably mediated through interactions at the myocardial cell surface. © 1986.