The effects of fenoterol and salbutamol on isometric force of contraction were studied in isolated, electrically driven human papillary muscle preparations. Fenoterol increased force of contraction at concentrations of 1 mumol l-1 and higher. The maximally effective concentration of fenoterol (100 mumol l-1) increased force of contraction by about 130%. The positive inotropic effect of fenoterol was not influenced by 0.1 mumol l-1 prazosin. The beta 1-selective antagonist atenolol (2 mumol l-1) and the beta 2-selective antagonist ICI 118551 (1 mumol l-1) shifted the concentration-response curve of fenoterol to the right, indicating that beta 1- and beta 2-adrenoceptors may contribute to the positive inotropic effect of fenoterol. In contrast to fenoterol, salbutamol increased force of contraction only by about 11% at 100 mumol l-1. The results indicate that: (1) fenoterol exerts a direct positive inotropic effect in the human heart which may support the beneficial effects of the reduction of systemic vascular resistance in patients with congestive heart failure; (2) this positive inotropic effect of fenoterol is mediated by beta 1- and beta 2-adrenoceptors; (3) the clinically observed improvement of cardiac performance in the case of salbutamol is presumably not due to any direct positive inotropic effect.