We investigated the pharmacokinetics of non-metabolized cefotaxime in 10 patients undergoing CAPD. The elimination half-life after IV administration of I g cefotaxime was 3.1 ± 1.3 hr, i.e. two to three times longer than in individuals with normal renal function but similar to patients with severe renal insufficiency. An average of 2.18% of the dose was recovered in the effluent. The halflife of I g cefotaxime administered in the dialysis solutions was 1.4 ± 0.8 hr. This difference between the half-lives after intraperitoneal and intravenous administration indicates a faster transport through the peritoneal membrane. Intraperitoneally administered cefotaxime -250 mg four times daily, was effective in the treatment of peritonitis in three CAPD patients. Since its introduction in 1976, CAPD has become an effective therapy for end-stage renal disease. The most serious complication is peritonitis and effective treatment is essential. Cefotaxime, a new broad-spectrurn cephalosporin, is active against most gramnegative and gram-positive organisms. It possesses no nephrotoxicity and may be useful in the treatment of peritonitis and other infections in patients on CAPD. This study was done to evaluate the pharmacokinetics of cefotaxime administered intravenously and intraperitoneally during CAPD.