n-Butyl maleate, also referred to as monobutyl maleate, is an ester of maleic acid, which is used as a counterion in the pharmaceutical industry. While substantial published data exist on short-term treatment, maleic acid-induced renal toxicity in the rat, no toxicity data are available on the monobutyl ester. This study evaluated the oral subchronic nephrotoxicity potential of n-butyl maleate administered to Sprague-Dawley rats (10/males and females/group) at doses of 0 (vehicle control), 10, 30, or 60 mg/kg/d for 2 wk. Statistically significant elevations in organ weights were noted in males at 60 mg/kg/d and included: (a) increases in absolute heart, kidney, and liver weights; (b) increased liver to body weight ratios; and (c) increased heart, kidney, liver, spleen, and epididymides to brain weight ratios. In females, statistically significant increases in organ weights were limited to increases in adrenal to brain weights at > or = 10 mg/kg/d, kidney to brain weights at > or = 30 mg/kg/d, and kidney to body weight and liver to brain weight ratios at 60 mg/kg/d. There were no macroscopic or microscopic pathology changes observed in any of the tissues examined. Importantly, light microscopic examination of the kidney was unremarkable at the end of the 2-wk dosing period with n-butyl maleate. Although lacking a histopathological correlate, resultant increases in organ weights at 60 mg/kg/d might be considered indicative of an adverse effect. However, renal perturbation induced by n-butyl maleate was mild in comparison to maleic acid-induced renal toxicity, which manifested as impaired tubular resorption and necrosis of the proximal tubules at doses > or = 60 mg/kg/d. The no-observed-adverse-effect level (NOAEL) for the study was 30 mg/kg/d.