In patients with chronic obstructive airway disease, the elimination half-lives of tetroxoprim (TXP) and sulfadiazine (SDZ) were found to be 8.6-9.0 h and 7.9-8.5 h, respectively, after administration of a single dose. The corresponding values for trimethoprim (TMP) and sulfamethoxazole (SMZ) were found to be 13.7 and 15.7 h, respectively. In the case of therapeutic multiple dosing, TMP and SMZ accumulate in patients' serum (tau = 12 h). Over a 10-day period of investigation, the serum levels measured were used to calculate half-life values of 12.9 h for TMP and 10.7 h for SMZ. Under the conditions of steady state, half-lives of 5.0 and 5.6 h were calculated for TXP and SDZ, respectively, which in the case of TXP might be explained kinetically. Due to the parallel change in the elimination-rate constants, no accumulation of TXP and SDZ in patients' serum (tau = 12 h) can occur.