1 The effects of food on the pharmacokinetics of orally and intravenously administered labetalol were assessed in six healthy volunteers. 2 Food caused a 38% increase in the mean systemic bioavailability of labetalol: from 0.26 +/- 0.03 (mean +/- s.e. mean) to 0.36 +/- 0.05 (P less than 0.05). 3 Food caused 20% increases in the volume of distribution of labetalol: (from 567 +/- 63 to 685 +/- 76) and in plasma clearance (from 1923 +/- 417 to 2344 +/- 566 ml/min. These changes did not reach statistical significance. Food did not alter the plasma half-life of labetalol after oral or intravenous dosing. 4 Food appears to increase labetalol bioavailability by decreasing its first pass metabolism. It is suggested that this change results from transient alteration of hepatic blood flow. The drug should therefore be taken at a standard time in relation to meals to minimise intraindividual variation in bioavailability.