We investigated the effect of peripheral serotonin receptor blockade on preeclamptic hypertension in 20 postpartum patients by the use of ketanserin, a serotonin receptor antagonist. In a study consisting of a double-blind crossover with placebo, parenteral ketanserin significantly reduced blood pressure from 167/105 to 126/71 mm Hg compared to a decline from 157/98 to 150/91 mm Hg for the placebo (p less than 0.001). All patients became hypertensive again following infusion, although no abrupt rebound in pressure occurred. Side effects were minimal. The results demonstrate that preeclamptic hypertension can be controlled by ketanserin and suggest that serotonin may have a role in the modulation of preeclampsia.