Pregnant adult C 57 BL/6 J strain mice, randomly assigned to 1 of 4 experimental groups, were irradiated with exponentially decreasing doses of tritium beta-rays but group 1 (used as a control) by single injection of tritiated water(HTO) at their 12.5 th day of gestation. Offsprings of male, received accumulative doses of 0, 0.5, 0.10 or 0.30 Gy in uterus were trained or examined on learning and memory ability or with behavioral tests. Significant dose-response relationships for alternations in those tests were found due to exposure to 0.10 Gy or above. These results indicate that exposure to HTO during the fetal period in mice results in dose-dependent alteration in postnatal behavior, learning and memory ability. 0.05—0.10 Gy exposure may represent a threshold for the experimental conditions of this research using these parameters.