Positive chronotropic as well as inotropic effects of 5-hydroxytryptamine (5-HT) have been observed in pig atrial tissue, but no data are available about the direct effects of 5-HT on ventricular tissue. In the present study we investigated inotropic effects of 5-HT on atrial and ventricular trabeculae obtained from hearts of 3 months old pigs. The baseline isometric contractile force was significantly higher in ventricular (4.14 +/- 1.25 mN) than in atrial tissue (0.47 +/- 0.11 mN). A noradrenaline concentration-response curve (0.01 to 10 mumol/l) was used to check contractile responsiveness of the tissue and all responses were expressed as percentage of the response to 10 mumol/l noradrenaline. Noradrenaline caused a concentration-dependent increase in contractile force in both atrial and ventricular trabeculae. In contrast, though 5-HT (0.01 to 100 mumol/l) did increase force of contraction in atrial tissue (maximum: 72 +/- 20% of the response to noradrenaline 10 mumol/l), the contractility of ventricular trabeculae was not significantly affected (maximum: 12 +/- 6%). The present data show that, in contrast to atrial tissue, contractile force of ventricular tissue could not be significantly affected by 5-HT. To our knowledge, this is the first study to show that an agent which increased force of contraction in the atrium, did not have a corresponding effect on the ventricle. These findings may have important implications for a better understanding of the physiology and pharmacology of cardiac contractility.