Previous studies in humans and rabbits demonstrated that methylprednisolone appears in the lung in greater concentration than prednisolone. To ascertain which pharmacokinetic properties of these drugs explain this difference, we gave methylprednisolone and prednisolone, 5 mg/kg intravenous bolus, to 23 adult rabbits. To measure the plasma concentration versus time curves for methylprednisolone and prednisolone, samples were obtained predose through 480 min postdose. To measure the bronchoalveolar lavage glucocorticoid concentration versus time curves, lavage was performed once per experiment at seven separate time points from 5 to 480 min post-dose (two to four experiments per time point). Bronchoalveolar lavage fluid (BALF) recovery ranged from 50 to 75% and was similar in both groups. Glucocorticoid concentration in plasma and BALF was determined by high performance liquid chromatography. To normalize for the dilutional effects of lavage, epithelial lining fluid (ELF) recovery was quantitated from BALF volume and BALF and plasma urea concentrations. Pharmacokinetic parameters of the two glucocorticoids were calculated both noncompartmentally and by a three-compartment model. The extrapolated plasma glucocorticoid concentrations at time zero of methylprednisolone and prednisolone are similar, but the volume of distribution and plasma half-life of methylprednisolone are significantly greater than those of prednisolone (p less than 0.05). Although the clearance of the two drugs are not significantly different, methylprednisolone appears to have a slower Cl than prednisolone. The mean residence time (the average time drug remained in body) was significantly longer for methylprednisolone than for prednisolone (p less than 0.05), and plasma glucocorticoid concentrations became significantly different in the two groups by 90 min (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)