Changes in transmembrane sodium fluxes have been reported in normotensive and in hypertensive subjects after ketanserin administration. In this study, the effects of the serotonergic system on transmembrane sodium transport mechanisms have been investigated in vitro. In erythrocytes drawn from ten healthy subjects, we studied the effects of serotonin (5HT) on the Na/K pump, Na/K cotransport, Na/Li countertransport, and passive permeability of Na. No significant changes were found. A direct, non-receptor-mediated action of ketanserin was then suspected, and the effects of two concentrations of ketanserin (5 x 10(-8) and 5 x 10(-7) M) were evaluated in erythrocytes from 12 normal volunteers. Both concentrations of ketanserin significantly decreased the activity of the Na/K pump and increased the activity of Na/Li countertransport. Na/K cotransport and passive permeability were not affected. Indirect evidence of the action of ketanserin on sodium transmembrane fluxes came from other experiments. In the red blood cells taken from five normal subjects and incubated for 2 hours in a plasma pool, we evaluated the changes in intracellular sodium concentration induced by the presence of 5HT or ketanserin. A significant decrease in intracellular sodium concentration occurred only with ketanserin. This study indicates that ketanserin has a direct influence on transmembrane sodium fluxes. If this action were also present in other cells, it might contribute to the actions of the drug at vascular, nervous, and renal tubular levels.