Pregnant ICR mice were immersed in water at 42 or 43 °C for 10 min once or twice daily on days 12 through 15 of gestation and the postnatal development of their offspring was observed. Postnatal weight gain of the offspring and their brain weight at 11 weeks of age were smaller than the values for control animals. As compared with controls, prenatally heated mice were less active in an open field and learned slower in a water-filled multiple T-maze and in a shuttle box. Thus, prenatal brief hyperthermia in mice was shown to suppress the body and brain growth during the postnatal period and adversely affect their emotionality and learning capacity.