The antihypertensive effects of intravenously and orally administered tiapamil and placebo were compared in 6 male patients (60 ± 5 years; mean ± SD) with mild essential hypertension (WHO classification Stage I) in a randomized double-blind crossover procedure. In the intravenous study, 1 mg/kg loading dose of tiapamil (or placebo) was followed by 50 µg/kg/min tiapamil or placebo over 2 h. In the oral study, the patients received placebo or approximately 6 mg/kg tiapamil. Blood pressure (BP) was determined by the Hawksley Random Zero Sphygmomanometer. Heart rate (HR) and P-R interval were derived from ECGs. Statistical significance was assessed by the paired Student’s t test. While intravenously administered placebo transiently increased both BP and HR, intravenous tiapamil significantly (p v≤ 0.05) lowered both variables. The P-R interval was also transiently but significantly (p ≤ 0.05) increased by intravenous tiapamil. Similarly, orally administered tiapamil showed an antihypertensive effect which was not observed with placebo. Neither HR nor P-R interval was influenced by oral tiapamil. The results suggest that tiapamil is an effective but short-acting antihypertensive agent.