Although 5-hydroxytryptamine (5-HT) elicits positive inotropic effects in the human isolated atrium via 5-HT₄ receptors, no data are available about its effects in the ventricular myocardium. We investigated the inotropic effects of 5-HT in human healthy ventricular trabeculae and compared these effects with those in right atrial trabeculae. Baseline force of contraction as (391 ± 10 and 719 ± 126 mg, respectively_than in atrial tissue (189 ± 5 and 383 ± 79 mg, respectively). However, 5-HT increased ventricular tissue. We conclude that, in contrast to atrial tissue, 5-HT is ineffective as a positive inotropic agent in human ventricular trabeculae. This finding obviously rules out the development of 5-HT₄ receptor agonists for the treatment of heart failure, but suggests the absence of ventricular side-effects of 5-HT₄ receptor (ant)agonists should these agents be used in the treatment of gastrointestinal disorders.