Phensuximide is a succinimide antiepileptic drug useful in the treatment of petit ma1 epilepsy. Phensuximide has been reported to be nephrotoxic in man but not in animals. In the present study, the effects of single and subacute administration for seven days of phensuximide on renal function and urinary tract morphology were evaluated in Sprague-Dawley and Fischer 344 rats. Single administration of phensuximide (1.25 mmol/kg, ip) induced mild changes in renal function (trace hematuria, increased proteinuria and decreased p-aminohippurate uptake). No morphological changes were observed at 24 hr. Subacute administration of phensuxirnide (0.6 mmol/kg/day, ip) produced diuresis in the Sprague-Dawley rat, but little functional evidence of nephrotoxicity. Renal morphological changes in Sprague-Dawley rats were seen primarily in distal segments of the nephrons. These changes were characterized by distensions of the basal infoldings, apical protrusions, and occlusion of some lumen. In the Fischer 344 rat, subacute phensuximide administration (0.3 or 0.6 mmol/kg/day, ip) resulted in transient hematuria and proteinuria, but no change in the other renal function parameters studied. Renal morphological changes observed in Fischer 344 rats occurred primarily in proximal tubular cells. Damaged cells were characterized by large vacuoles at the basal infoldings, accumulations of opaque granules, migration of nuclei to the lumenal membranes, occlusion of the lumen and/or loss of the brush border. Morphological damage was more widespread in Fischer 344 rats than in Sprague-Dawley rats. Areas of focal hemorrhage were observed in ureters and bladders, especially in Fischer 344 rats given phensuximide 0.6 mmol/kg/day. However, morphological effects may be reversible with continued dosing. It was concluded that phensuximide produces mild, transient renal effects in Fischer 344 rats, and that the Fischer 344 rat is a suitable model for studying phensuximide-induced toxicity to the urinary tract.