The effect of phenobarbitone on the steady state volume of distribution (Vdss) and the total body blood clearance (CLtot, b) of imipramine and the serum concentration of its metabolite, desipramine was examined. The serum disappearance of imipramine after an 8 mg kg-1 i.v. dose followed a biexponential decline in both control and phenobarbitone-treated rats while the concentration of its metabolite increased in the phenobarbitone-treated rats then rapidly declined compared with that in control rats. Since CLtot,b was nearly equal to the hepatic blood flow (QH), QH may be the rate-determining step of imipramine elimination. In the control rats the Vdss of imipramine was large at 19.9 litre kg-1. In the phenobarbitone-treated rats the pharmacokinetic parameters, biological half-life (t1/2) and Vdss significantly decreased to approximately 23-40% while CLtot,b increased to 126% of those in the control rats, although the latter difference was not statistically significant. The blood-to-plasma concentration ratios (RB) of imipramine and desipramine decreased in the phenobarbitone-treated rats. The urinary excretion ratios of imipramine and desipramine, to the dose of imipramine over 8 h, were less than 1.5% in both groups. These ratios were not significantly changed in the phenobarbitone-treated rats. It was concluded that the significant decrease in t1/2 of the phenobarbitone-treated rats may not be attributed to the changes in CLtot,b and/or in the urinary excretion, but mainly to the decrease in Vdss.