The peritoneal pharmacokinetics of gentamicin were investigated in patients on continuous ambulatory peritoneal dialysis. Gentamicin was added to the dialyzate, and gentamicin concentrations were measured repeatedly in serum and dialyzate. After administration of a single dose of gentamicin, equilibrium between serum and dialyzate levels is achieved within 24 hours; dialyzate levels of gentamicin are lower and mass transfer coefficients higher in patients with peritonitis. With chronic intraperitoneal gentamicin treatment for peritonitis, steady state serum levels around 4 to 5 mg/l are achieved and otovestibular toxicity should be feared.