1. The pharmacokinetics of methylprednisolone (MP) were studied in five normal subjects following intravenous doses of 20, 40 and 80 mg methylprednisolone sodium succinate (MPSS) and an oral dose of 20 mg methylprednisolone as 4 x 5 mg tablets. Plasma concentrations of MP and MPSS were measured by both high performance thin layer (h.p.t.l.c.) and high pressure liquid chromatography (h.p.l.c.). 2. The mean values (+/- s.d.) of half-life, mean residence time (MRT), systemic clearance (CL) and volume of distribution at steady state (Vss) of MP following intravenous administration were 1.93 +/- 0.35 h, 3.50 +/- 1.01 h, 0.45 +/- 0.12 lh-1 kg-1 and 1.5 +/- 0.63 1 kg-1, respectively. There was no evidence of dose-related changes in these values. The plasma MP concentration-time curves were superimposable when normalized for dose. 3. The bioavailability of methylprednisolone from the 20 mg tablet was 0.82 +/- 0.11 (s.d.). 4. In vivo hydrolysis of MPSS was rapid with a half-life of 4.14 +/- 1.62 (s.d.) min, and was independent of dose. In contrast, in vitro hydrolysis in plasma, whole blood and red blood cells was slow; the process continuing for more than 7 days. Sodium fluoride did not prevent the hydrolysis of MPSS.