The pharmacokinetics of tobramycin and clindamycin were examined in patients undergoing peritoneal dialysis for chronic renal failure. Peak serum levels of tobramycin in functionally anephric patients were less than expected, probably secondary to a larger volume of distribution. Peritoneal dialysis resulted in a significant clearance of tobramycin, with a resultant reduction in serum half-life. The present data suggest that, if bactericidal serum levels of tobramycin are to be maintained in patients undergoing peritoneal dialysis, a parenteral loading dose be administered, followed by either (i) an identical dose every third half-life or (ii) one-half the loading dose every half-life. However, optimal therapy is best achieved by monitoring serum levels to insure appropriate drug dosage. Peak serum levels of clindamycin in functionally anephric patients were approximately twofold greater than those expected in normals after an identical parenteral dose. It is, therefore, recommended that the administered dose of this agent in functionally anephric patients be one-half of that required to produce desired peak serum levels in patients without renal impairment. Peritoneal clearance of clindamycin during dialysis was shown to be essentially zero, indicating that dialysis does not affect clindamycin disposition.