The kinetics of moxalactam has been investigated in 10 subjects undergoing continuous ambulatory peritoneal dialysis (CAPD). A single 1 g dose was injected i.v. and a 1 g dose was given intraperitoneally in the CAPD fluid during a 4 h dwell-time. Moxalactam was assayed by HPLC. After i.v. injection, the serum kinetics of moxalactam were: plasma t 1/2 = 17.9 h; volume of distribution at steady-state, 0.27 l/kg; total plasma clearance, 12.8 ml/min; peritoneal clearance, 2.1 ml/min. Dialysate moxalactam concentrations rose rapidly but only 20% of the dose was eliminated by the peritoneal route. After intraperitoneal instillation, moxalactam appeared in the serum rapidly and the peak serum concentration ranged from 21 to 49 micrograms/ml after between 4 and 5 h. The absorption of moxalactam from the peritoneal space was 57 +/- 16%. The data suggest that moxalactam has bidirectional exchange characteristics through the peritoneal membrane. Instillation of moxalactam in CAPD fluid may permit rapid absorption and the appearance of a therapeutic serum concentration.