Male Sprague Dawley rats were treated with buthionine sulfoximine (BSO) and cisplatin in different doses and schedules to optimize the chemoprotective effect of BSO against cisplatin nephrotoxicity. BSO at 4 mmol/kg, administered s.c. 2 h prior to cisplatin, resulted in normal blood urea nitrogen (BUN) levels in rats treated with 3 or 4 mg/kg cisplatin, and modestly, but significantly reduced the toxicity of cisplatin at 5 mg/kg. Administration of BSO (4 mmol/kg) at intervals ranging from 0 to 16 h prior to cisplatin (5 mg/kg) resulted in a significant reduction in BUN values. A BSO dose as low as 0.04 mmol/kg was found to be as effective as 4 mmol/kg against nephrotoxicity associated with cisplatin at 4 mg/kg. Repetitive injections of BSO (1 mmol/kg every 12 h, four times, beginning 2 h prior to cisplatin) significantly inhibited elevations of BUN associated with higher-dose cisplatin (6 mg/kg), whereas a single BSO injection of 4 mmol/kg was ineffective. The degree and duration of renal glutathione depletion was related to the dose of BSO. Renal glutathione content following 4 mmol/kg BSO was 38% of control at 2 h and 40% at 24 h; following 0.04 mg/kg, glutathione was 47% at 2 h and almost 100% at 24 h. Simultaneous in vitro administration of BSO did not inactivate cisplatin cytotoxicity as measured by the colony-forming ability of MBT-2 cells in soft agar. These data indicate that repeated injections of BSO, beginning prior to cisplatin administration, would improve the nephroprotective effect without compromising the chemotherapeutic efficacy of cisplatin. It is suggested that the ability of BSO to reduce cisplatin nephrotoxicity may not correlate with the degree of renal glutathione depletion and that the mechanism of action is unlikely to involve direct inactivation of cisplatin.