In an open cross-over experiment, the influence of the antimicrobial agent co-trimoxazole on the single-dose pharmacokinetics of theophylline was studied in six healthy adults by comparing the pharmacokinetic parameters found after intravenous administration of theophylline without and with co-medication of co-trimoxazole for the previous 8 d. Theophylline concentrations in plasma were measured by high-performance liquid chromatography (HPLC) analysis. During each treatment, a concentration-time curve was evaluated. No influence of co-trimoxazole on the rate of elimination and volume of distribution of theophylline could be found, as a result of which theophylline concentrations in plasma were not significantly different in both periods of drug administration. A similar lack of influence of co-trimoxazole may apply to the steady-state pharmacokinetics of theophylline. The present study suggests that both drugs can be given concomitantly without the need for dosage adjustment of theophylline.