Oral administration of hydroquinone (HQ) over 2 years to male Fischer 344 (F344) rats results in a dose-related nephropathy and an increase in the incidence of renal tubule adenomas. Female F344 rats, B6C3F1 mice, and Sprague-Dawley (SD) rats are resistant to the chronic renal toxicity of HQ, and nephrotoxicity was not seen in dogs or humans following subchronic exposure. To better characterize the early development of renal toxicity in rats, cell proliferation was quantitated within the proximal (P1, P2, and P3) and distal tubule segments of the kidney in rats given 0, 2.5, 25, or 50 mg/kg HQ by gavage. Male and female F344 rats were treated for 1, 3, or 6 weeks, and male SD rats were treated for 6 weeks. Cell proliferation was quantitated by incorporation of bromodeoxyuridine, detected immunohistochemically, into newly synthesized DNA. At 6 weeks, renal cell proliferation was increased over vehicle-controls in male F344 rats dosed at 50 mg/kg. Significant elevations (p < 0.001) occurred in the P1 segments (87%) and in the P2 segments (50%) but the elevation in the P3 segment (34%) was not statistically significant. Urinalyses revealed increases in the rate of excretion of enzymes indicative of proximal tubular damage. Histopathologic evaluation of the kidneys was consistent with a dose-related tubular degeneration in the male F344 rat. No chemical-related effects were observed in the kidneys of female F344 and male SD rats. These data parallel the findings of sex- and strain-specific kidney adenomas in the 2-year bioassays, and suggest that chemically induced cell proliferation secondary to toxicity may be important in the pathogenesis of benign renal tumors in male F344 rats treated with HQ.