BACKGROUND: In some studies, 5 microgram/kg clonidine premedication was claimed to enhance the pressor effects of ephedrine in anesthetized patients. We studied hemodynamic responses to intravenous responses, themselves responses to intravenous ephedrine in patients who received clonidine 3 microgram/kg. METHODS: 40 ASA pysical status I or II patients were randomly assigned to either the clonidine group (n = 20), receiving oral clonidine 3 microgram/kg 90 min before general anesthesia, or the control group (n = 20), receiving no clonidine. Hemodynamic measurements were made at one-minute intervals for ten minutes after ephedrine 0.1 mg/kg was injected as a bolus. RESULTS: The magnitude of maximal systolic blood pressure increases in the clonidine group (13.2+/-9.3%) was no different in the control group (12.4+/-12.3%). There were no difference in the pressor effect and duration of response of ephedrine in both groups. CONCLUSIONS: The pressor effect of ephedrine is not enhanced in patients given 3 microgram/kg clonidine premedication during general anesthesia.