Based on results of studies of a possible terfenadine-theophylline interaction on file with Merrell Dow, Inc., a randomized, crossover study was undertaken to determine if there were a pharmacokinetic interaction between these two medications. Seventeen normal volunteers were randomized to receive theophylline or a theophylline-terfenadine combination for 14 days, followed by a 2-week washout period, with crossover to the other treatment for 14 days. Serum theophylline concentrations were measured on the last day of each treatment phase for determination of pharmacokinetic parameters. No differences were found in elimination rate constant, elimination half-life, area under the concentration-time curve, time to peak concentration, or maximum theophylline concentration. It is concluded that concomitant terfenadine therapy does not affect the pharmacokinetic profile of theophylline in healthy male volunteers.