Summary:The absolute bioavailability of oral rifampin was determined in 20 pediatric patients. Intravenous doses of rifampin (mean 287 mg/m2) were compared with p.o. doses (mean 324 mg/m2). Serum concentrations of rifampin, 25‐O‐desacetylrifampicin, and 3‐formylrifamycin SV were determined by high performance liquid chromatography. Following a ½‐h intravenous infusion, serum rifampin concentrations declined in a monoexponential fashion. Pharmacokinetic analysis of the rifampin serum concentration data indicated that only 50 ± 22% of a freshly prepared p.o. suspension was absorbed. The rifampin elimination half‐life following i.v. administration (2.25 ± 0.64 h) was not different from that observed following p.o. dose administration (2.61 ± 1.35 h). Peak rifampin concentrations were significantly higher following i.v. administration when corrected to a 300 mg/m2dose (27.4 vs. 9.1 &mgr;g/ml, respectively, p < 0.0001) than after p.o administration. The peak concentration following a p.o. dose occurred at 2.0 ± 0.9 h. The ratio of desacetylrifampicin to rifampin areas under the curves were similar for i.v. and p.o. routes of administration (0.23 vs. 0.19), suggesting linear metabolism of rifampin to this metabolite. 3‐formylrifamycin SV concentrations were lower than those of desacetylrifampicin and were detectable in less than half of the patients. The results of this study indicate the need for larger p.o. doses when serum concentrations similar to those obtained following intravenous doses are desired.