The concentrations of tinidazole in various tissues and body fluids were studied in gynaecological patients after a single 2 g oral dose. Tinidazole was determined by the agar-diffusion technique using a strain of Clostridium bifermentans. Reliable estimates of concentrations down to 0.5 μg/ml could be obtained. Dichloromethane extraction of tinidazole added to various tissues in known amounts gave a recovery of 100 ± 10%. Peak serum values of 32–52 μg/ml were reached 3–6 h after the administration. The concentrations in peritoneal fluid, obtained at operation 8.5–15 h after the intake, varied between 16 and 40 μg/ml. Specimens from the Fallopian tubes yielded 15–26 μg tinidazole/g tissue; similar levels were obtained in specimens from myometrium, endometrium, portio, vaginal secretions, omental fat, and cutis. It is concluded that, with the given dose, tinidazole concentrations are achieved in fluids and tissues of the female genital tract that are far in excess of those that should be therapeutical in infections caused by microorganisms known to respond to nitroimidazo