Long-Evans rats, 4-6 weeks of age, were dosed with 0.0, 2.5, 5.0, or 10.0 ml/kg chlorine dioxide (ClO2) in deionized water for up to 73 days. Males were exposed for 56 days and females for 14 days prior to breeding, and throughout the 10-day breeding period. Males were killed and evaluated for sperm parameters and reproductive tract histopathology following the breeding period. Females continued to be dosed throughout gestation and lactation until weaning on lactation day 21, when both dams and selected pups were necropsied. Neither clinical signs of toxicity nor adverse effects on any reproductive parameter examined were observed in the parental animals. Litter size, pup viability, and pup weight were unaltered by chlorine dioxide exposure. F0 reproductive tract organ weights and F1 organ weights for testis, epididymis, uterus, and ovaries were not different between groups, but vaginal weight was significantly decreased (P less than 0.03) for female weanlings in the high dose (10.0 mg/kg) group relative to controls. There were no changes in thyroid hormone parameters that appeared to be attributable to chlorine dioxide treatment.