The pharmacokinetics of diclofenac sodium in healthy volunteers was evaluated to determine if previously repeated doses of ranitidine inhibited the metabolism of the non-steroidal anti-inflammatory drug. Diclofenac sodium 50 mg (tablets) in combination with ranitidine 150 mg (tablets) were administered to 14 healthy human volunteers in a two treatment study design, separated by 5 days in which the ranitidine alone was administrated in single p.o. doses twice daily. Plasma concentrations of diclofenac were determined during a 12 hour period following drug administration. Diclofenac plasma concentrations were determined by a validated RP-HPLC method. Pharmacokinetic parameters were calculated with compartmental and non-compartmental analysis. In the two periods of treatments, the mean peak plasma concentrations Cmax were 1503.9 ng/ml (diclofenac alone) and 1742.5 ng/ml (diclofenac and ranitidine). The time taken to reach the peak, Tmax, was 0.85 hrs, and 0.82 hrs, respectively. The areas under the curve (AUC0-6) were 1479.9 ng x hr/ml and 1650.3 ng x hr/ml, respectively. Statistically insignificant difference was observed in these pharmaco-kinetic parameters of diclofenac sodium when administered alone or after 5 days of treatment with ranitidine. The experimental data did not suggest any consistent effects of ranitidine upon the pharmacokinetics of diclofenac sodium.