Ketanserin has been shown to antagonise both alpha 1-adrenergic (alpha 1) and 5-HT2-serotonergic (5-HT2) receptors in animal experiments but the findings in humans have been conflicting. Its mode of action in reducing blood pressure is unclear. We have studied, therefore, the effects of ketanserin 40 mg b.i.d. given orally for 4 days to seven normal volunteers on blood pressure, heart rate, and tests of baroreflex function, including infusion of the alpha 1-adrenergic agonist, phenylephrine. Ketanserin caused a small reduction in both lying and standing blood pressure after 4 days but with no change in heart rate; no subject had a severe fall in pressure after the first dose. The 30:15 and Valsalva manoeuvre ratios were decreased slightly. The dose-response curve of blood pressure against rate of phenylephrine infusion was shifted to the right in keeping with alpha 1-adrenergic antagonism; the degree of shift was small compared with that after prazosin. The relationship between change in blood pressure and heart rate during phenylephrine infusion was not affected by ketanserin. Ketanserin may act as an alpha 1-adrenergic antagonist. The development of more selective agents to antagonise 5-HT2-serotonergic receptors should clarify the role of this aspect of the drug's action in reducing blood pressure.