The nephrotoxicity and auditory toxicity of high-dose (mean, 28.6 g) and prolonged (mean, 61.6 days) courses of gentamicin and tobramycin were monitored in 15 patients receiving 17 courses of treatment for pseudomonas endocarditis. Doses were adjusted in a manner that maintained peak levels of aminoglycoside in serum at 12-15 micrograms/ml and trough levels at less than 2 micrograms/ml. Drug-related renal dysfunction and auditory toxicity occurred in 63% and 44%, respectively, of gentamicin-treated patients and in 43% and 25%, respectively, of tobramycin-treated patients. Mean maximal rises (+/- SEM) in serum creatinine levels were 0.8 (+/- 0.4) mg/dl in the group given gentamicin and 1.6 (+/- 0.7) mg/dl in the group given tobramycin. Mean maximal decreases in pure-tone hearing threshold levels were greater in gentamicin-treated patients (58.3 dB) than in those given tobramycin (22.5 dB). Both forms of toxicity appeared earlier and at a smaller dose with gentamicin than with tobramycin.