Previous studies have shown a modulatory effect of sigma receptor activation on cardiac contractility and its desensitisation. These experiments were performed in isolated rat ventricular myocytes. Since the rat myocardium is well known for a number of quite specific properties, this study was aimed at verifying the effects of sigma ligands in guinea pig atria. Isometric contractions were recorded in 19 adult male guinea pigs (stimulated at the rate of 1Hz). After 30-45 minutes of stabilisation haloperidol in 10nM concentration was added to the bath for 30 minutes. Then, a period of 15 min washout and another 30 min period of haloperidol perfusion followed. The changes of amplitude of contraction and of mechanical restitution were investigated. In all cases, marked positive inotropic effect was observed. The mechanical restitution pattern was not altered. This positive inotropic effect was attenuated after the second haloperidol administration which could indicate that sigma receptors in guinea pig heart undergo a desensitisation process previously reported in rat myocardium. According to the effect on the mechanical restitution it is possible to speculate that it is mediated via increased availability of cytoplasmatic calcium.