The effects of 5-hydroxytryptamine (5-HT) on force of contraction (F_C), action potential (AP) and calcium current (I_Ca) were studied in human right atrial and left ventricular heart muscle. 5-HT exerted a concentration-dependent increase in F_C in multicellular atrial preparations; the EC₅₀ was approximately 3 × 10⁻⁷ mol/l. Maximal increases in F_C (252±58% of control values; means ± SEM, n=6) were obtained at 5-HT 10⁻⁵ mol/l. At this concentration, I_Ca was increased four- to sevenfold in enzymatically isolated atrial myocytes. In contrast, ventricular preparations did not respond to 5-HT; F_C, AP and I_Ca remained unaffected. In the same preparations, F_C was increased by isoprenaline three- to fourfold. These results confirm the observation that 5-HT induces a positive inotropic effect in the human atrium, possibly mediated by activation of the adenylyl cyclase — cyclic AMP system. Our study demonstrates, however, the complete lack of functional 5-HT receptors, with respect to changes in F_C, in the human ventricle. Since the positive inotropic effect of 5-HT in the human heart is obviously restricted to the atrium, our findings question the concept of developing 5-HT receptor agonists for the treatment of heart failure.