The pharmacokinetics of theophylline and methylprednisolone were examined before and during cimetidine treatment in an asthmatic woman who required long-term administration of these drugs. Cimetidine reduced theophylline plasma clearance by 30 percent, and measurement of urinary metabolites showed that 3-methylxanthine formation was inhibited more strongly than that of the methylated uric acid metabolites. Assessment of methylprednisolone disposition following oral and intravenous doses revealed no effect of cimetidine on the bioavailability (74 to 81 percent absorption) or plasma clearance (22 to 24 liters per hour) of the steroid. Thus, cimetidine exhibits variable and selective effects on the biotransformation pathways of drugs important in asthma therapy.