The potential influence of pantoprazole (BY1023/SK&F96022), a newly developed selective inhibitor of the gastric H+,K(+)-ATPase, on therapeutic serum theophylline concentrations was investigated in a crossover study in 8 healthy male volunteers (age 25-30 [median 27] years, body weight 63-80 [median 68] kg). Steady-state serum theophylline concentrations were obtained by a two-step intravenous infusion scheme of approximately 350 mg theophylline each over 0.5 h and subsequently over approximately 10 h, respectively. In the test period, 30 mg pantoprazole were injected over 2 min on 5 consecutive days and theophylline was infused on day 4. In the reference period, placebo was administered i.v. on 2 consecutive days and theophylline on day 1. Serum pantoprazole concentrations were measured up to 12 h, serum theophylline concentrations up to 36 h. Pantoprazole was well tolerated with and without theophylline. There were no clinically relevant changes in blood pressure, heart rate, ECG and routine clinical laboratory parameters. Primary characteristic for confirmative assessment of no interaction was the area under the concentration/time curve (AUC). Lack of interaction in the sense of equivalence was concluded both for theophylline (with and without pantoprazole) and pantoprazole (with and without theophylline), as the 90%-confidence intervals of the AUC-ratio test/reference were within the equivalence range of 0.8 to 1.25. Further explorative analysis of theophylline disposition kinetics revealed this inclusion also for clearance and volume of distribution, but not for the half-life. In the case of pantoprazole, the corresponding 90%-confidence intervals for any of the secondary characteristics clearance, volume of distribution and half-life were within the above mentioned range. In conclusion, repeated once-daily i.v. injections of 30 mg pantoprazole have no clinically relevant influence on steady-state theophylline serum concentrations, nor does theophylline at therapeutic serum concentrations influence the pantoprazole disposition kinetics. Hence, in clinical practice theophylline and pantoprazole can be administered concomitantly without dose adjustment.