The pharmacokinetics of doxycycline and cephalothin in genital tissues and serum were evaluated in 38 patients undergoing hysterectomy. Antibiotic concentrations in the ovary, fallopian tube, myometrium, cervix, and endometrium were measured at varying intervals after antibiotic administration. Cephalothin concentrations in tissues were critically dependent on the time interval between preoperative antibiotic administration and removal of the genital tract tissues, whereas doxycycline concentrations in tissues were consistently reproducible in the time frame evaluated (I to 4| hours after drug administration). Doxycycline endometrial concentrations were consistently higher than those measured elsewhere in the genital tract (P<0.05). These results are of importance in a reassessment of traditional approaches to antibiotic use in gynecologic surgery and in the management of pelvic inflammatory disease (PID).