The oral and intravenous pharmacokinetics of labetalol were determined in five subjects before and after a 3 week course of glutethimide 500 mg/day. After glutethimide there was a significant reduction in the AUC after the oral dose of labetalol, from 40,596 +/- 11,534 (mean +/- s.e. mean) to 22,057 +/- 6,276 ng ml-1 min (2P less than 0.05), and systemic bioavailability was reduced from 30.3 +/- 2.8 to 17.0 +/- 3.5% (2P less than 0.001). There was no significant change in labetalol plasma concentration-time curve (AUC) following an intravenous dose, half-life, volume of distribution, and plasma clearance. The oral and intravenous pharmacokinetics of labetalol were determined in six subjects before and after a 3 day course of cimetidine 1.6 g/day. After cimetidine there was a significant reduction in the volume of distribution of labetalol, from 520 +/- 51 to 445 +/- 24 1 (2P less than 0.05). The AUC of labetalol after the oral dose increased by 66%, from 51,029 +/- 7,950 to 84,772 +/- 19,444 ng ml-1 min (2P = 0.06). The systemic bioavailability of labetalol increased from 25.1 +/- 2.4 to 39.0 +/- 7.6% (2P = 0.06). There was no significant change in labetalol AUC after the intravenous dose, half life, and plasma clearance. There were no significant changes in resting heart rate and supine systolic and diastolic blood pressure following labetalol plus glutethimide, or labetalol plus cimetidine.(ABSTRACT TRUNCATED AT 250 WORDS)