Ketanserin, which preferentially blocks 5-HT2-serotonergic receptors, was injected intravenously (i.v.) to patients with congestive heart failure in a bolus dose of 10 mg, followed by an i.v. infusion of 3 mg/h over a period of 4 h. The drug caused a decrease in total peripheral resistance and, conversely, an increase in stroke volume. Right atrial and pulmonary artery pressures were decreased. Plasma noradrenaline rose twofold over the basal levels shortly after injection, but showed a distinct fall 2 h after beginning of the treatment. The concentrations of ketanserin in plasma after bolus injection approximated 100-150 ng/ml. The sympathoneuronal and sympathoadrenal reaction during tilting were increased after i.v. injection of 10 mg ketanserin in volunteers. The noradrenaline and adrenaline levels in plasma rose significantly more when compared with values before the injection of the drug. In vitro as well as in vivo ketanserin exerts a concentration-dependent inhibitory effect on the active transport of serotonin and catecholamines (dopamine, noradrenaline, adrenaline) into human platelets. Considering platelets as a model of the sympathetic neurons, the inhibition of reuptake of catecholamines by ketanserin could contribute to the observed increase in circulating catecholamines after injection of the drug.