The time courses of plasma metoclopramide concentrations were followed in six subjects after oral and intravenous single dose administration. Plasma concentration-time data following i. v. administration in each subject were found to fit a two compartment model with a mean terminal half-life of 4.55 h +/- 0.80 h and a mean distribution half-time of 0.35 h +/- 0.09 h. Volumes of distribution were high (3.43 +/- 1.181 . kg-1), and clearances (0.53 +/- 0.191 . kg-1 h-1) approached liver plasma flow. This suggests that metoclopramide occurs at higher concentrations in tissues than in plasma, and that its clearance is probably limited by liver blood flow rather than liver metabolic capacity. The postabsorption decline in metoclopramide plasma levels after oral administration was also biexponential in each subject. The terminal half-life was 5.17 h +/- 0.98 h. Mean volume of distribution and mean clearance were similar to intravenous values (after adjustment for bioavailability). Oral absorption was rapid with peak plasma concentrations being reached at a mean time of 0.93 h. A mean bioavailability of 0.77 was calculated for the six subjects, and it was postulated that this incomplete availability is due to a first-pass effect. The inter-individual variation in the degree of "first-pass' was considerable (0.47--1.14).