Sulfadiazine (SD, 250 mg) or sulfamethoxazole (SM, 800 mg) in combination with trimethoprim (TMP, 160 mg) was given twice daily to male patients and the concentrations of these drugs were assayed simultaneously in plasma, urine and the male reproductive organs (prostate gland, epididymis and testis). The antibacterially active mean concentration of SD in the prostate was 0.33, in the epididymis 0.74 and testis 0.85 times that in the plasma. The respective values for SM were 0.21, 0.51 and 0.53 and for TMP 0.73--0.73, 179--2.00 and 1.90--2.23. This indicates that the penetration of TMP into these tissues is two-fourfold compared to that of these sulfonamides. The drug concentrations and the concentration ratios (SD/TMP, SM/TMP) obtained suggest that by standards proposed in the literature they are adequate for synergistic antibacterial action at the serum, urine and tissue levels. Although the amount of SD was only about 1/3 of that of SM it seems to be a comparable alternative with this small dosage in combination with TMP, when bacterial infections in the urinary tract or in the male reproductive organs are treated.