AbstractAs an attempt to establish the methodology of behavioral teratology, this study was carried out to investigate whether the effects of HU would be different between Wistar and Sprague‐Dawley (SD) rats. Hydroxyurea (HU) was intraperitoneally injected to pregnant SD rats at does of 50 or 100 mg/kg/day during the organogenetic period (days 9 to 12 of gestation). Male offspring from these dams were observed for their morphological and behavioral developments. Microphthalmia was found in only 2 pups at the 21st day of the 100 mg/kgyday group. No adverse effect of postnatal growth was observed in all treated groups. The number of ambulation in the open field test at 3 and 4 weeks of age in the 50 mg/kg/day group and at 5 weeks of age in the 100 mg/kg/day group was decreased. No inhibition of reflex development, traction response and rotorod performance was observed in all treated groups. These findings suggested that the morphological and behavioral effects of HU in SD male rats are less severe than those in Wistar male rats as was in our previous study. The strain differences in the exploratory behavior seemed to be more severe, because the specific behavioral type in both strains might be potentiated by HU treatmen