The effect of concurrent cimetidine administration on the disposition of pethidine was investigated in eight healthy male volunteers (18-31 years). The subjects received 70 mg i.v. pethidine HCl doses before and during cimetidine treatment (1200 mg/day p.o.). During cimetidine treatment, pethidine total body clearance (CL) decreased by 22% (0.611 +/- 0.101 [mean +/- s.d.] to 0.474 +/- 0.098 1 kg-1 h, P less than 0.05) and pethidine volume of distribution at steady state (Vss) decreased by 13% (4.79 +/- 0.82 to 4.16 +/- 0.75 l/kg, P less than 0.05). A cimetidine-induced reduction in pethidine oxidation to norpethidine was suggested by a 23% reduction in norpethidine area under the curve from 0 to 24 h (472 +/- 93 to 362 +/- 38 ng ml-1 h, P less than 0.05) and a 29% reduction in peak norpethidine concentration (26.7 +/- 5.3 to 18.9 +/- 1.9 ng/ml, P less than 0.05). There were no significant linear correlations of serum trough cimetidine concentration with percentage reductions in pethidine CL, pethidine Vss, norpethidine AUC (24), or norpethidine peak concentrations. It would appear that the cimetidine-pethidine kinetic interaction may be of sufficient magnitude to be clinically significant. Caution is advised when patients are treated concurrently with these two agents.