The influence of gender on the single-dose pharmacokinetics of oral ciprofloxacin was investigated in a parallel designed study in 24 healthy, fasted volunteers. Ciprofloxacin (750 mg) was administered as a single tablet to 10 women and 14 men. All subjects were Caucasian except for one African American man. The mean age was 36 years for men and 33 years for women. Women received the drug during the nonmenstruating phase of their menstrual cycle and were not taking oral contraceptives at the time of the study. Plasma samples were collected over a 24 h period and assayed by h.p.l.c. The 90% confidence limits for the ratio of geometric means of maximum plasma concentration, apparent terminal half-life, and apparent oral plasma clearance (corrected for body weight) were between 80% and 125%, and those for the area under the plasma concentration-time curve were 71% and 101%. Results suggest that there were no gender-related differences in ciprofloxacin pharmacokinetics in men and nonmenstruating women of middle age.