Gentamicin kinetics were determined after intravenous or intraperitoneal injection in five patients undergoing continuous ambulatory peritoneal dialysis (CAPD). Our objective was to determine rate of absorption of gentamicin from the peritoneum into the systemic circulation and vice versa. After intraperitoneal instillation of 1 mg/kg in the CAPD fluid during a 6-hr dwell time, the antibiotic appeared in the serum within 15 min in four of five patients. Peak serum concentrations ranged between 1.6 and 7.2 mg/l(mean +/- SD = 3.52 +/- 2.22) in all five patients and the time to reach peak concentration was 3.8 +/- 1.5 hr. Peritoneal gentamicin clearance was 13 ml/min. Percent extraction of gentamicin from the PD fluid within the 6 hr of intraperitoneal exposure ranged from 65% to 100% (mean +/- SD = 86.8 +/- 13.2). The fraction of the intraperitoneal dose absorbed into systemic circulation was found to be 0.84 independently by calculating the ratio of AUCip and AUCiv. When the same dose of gentamicin was injected intravenously (1 mg/kg), no gentamicin could be detected in the peritoneal fluid in three of five patients and only a very small amount of the drug was present for a brief period of time in the remaining two. The kinetic parameters of intravenous gentamicin were: volume of distribution, 0.3 l/kg; elimination rate constant, 0.028 hr(-1), plasma clearance 0.009 l/kg . min(-1), and half-life 27.4 hr. In two patients with acute peritonitis treated with intraperitoneal gentamicin, peak serum concentrations were found to range between 3.5 and 4.5 mg/l. These data suggest that gentamicin is rapidly absorbed from the peritoneal fluid into the blood compartment, but that occurrence of the reverse exchange is negligible. Thus, CAPD would not be expected to alter the elimination characteristics of intravenous gentamicin. Instillation of gentamicin in CAPD fluid may allow rapid absorption to reach therapeutic serum concentrations.