The mutagenicity of hydroquinone was evaluated in a dominant lethal assay using 4 groups of 25 male Charles River rats orally exposed by gavage to dose levels of 0, 30, 100 or 300 mg/kg/day (as a 5% water solutions) for 5 days/week over a 10 week period. Following exposure, each male was mated for 7 days/week with 1 untreated female/week for 2 consecutive weeks. Females were sacrificed on the 14th day of gestation. The mean body weights of the high-dose level males were significantly lower than controls from the 4th week to the end of the treatment period. Clinical signs of toxicity included brown colored urine for all treated animals and swollen eyelids, porphyrin-like tears, sialorrhea, spastic gait, tremors, convulsions, and spontaneous deaths (2 apparently treatment-related deaths) in the high-dose group. No statistically significant differences were observed between treated and control animals with respect to insemination rate, pregnancy rate, mean numbers of implantation sites, viable implants, early and late deaths, and pre- and post-implantation loss. Statistically greater mean numbers of corpora lutea and implantations/dam were observed in the high-dose group relative to the controls in the first mating period.