In a prospective study, the epididymal penetration of ceftriaxone was evaluated in order to use it in the treatment of orchi-epididymitis in men. A bolus intravenous dose of 1 g of ceftriaxone was administered to 15 patients hospitalized for surgery as part of treatment for prostatic adenoma or prostatic cancer. Nine successive blood samples were collected in the interval from 0 to 24 h after administration, and epididymis samples were taken 0.75 h after administration. Concentrations of drug in all samples were assayed by a reverse-phase-ion pairing high-performance liquid chromatography method with UV detection. The results showed that the pharmacokinetics of ceftriaxone in serum did not differ from those determined previously in healthy volunteers. The terminal half-life was 6.9& 1.7 h, and the mean residence time 9 -5 2 2 . 3 h. The volume of distribution was 0.144+0-018 Ikg-' and the total body clearance 1-17kO.29 1 h-I. The concentrations in tissue reached 27.2 & 6 pg g-' in right epididymis, and 25 a4 6.2 pg g-in left epididymis. The tissue-versus-serum concentration ratios ranged from 0.175 to 0.545 (mean value, 0.295 kO.099). The concentrations in serum and tissue observed in this study were in excess of the MICs for bacteria considered to be susceptible to ceftriaxone, particularly Neisseriu gonorrhoea and coliform bacteria.