Pregnant rats in 11d and 16d of their pregnancy were given one−off whole body exposure by ,137,Cs γ rays to 0.2, 0.4, 0.9 and 2.0Gy, respectively. Changes were observed in conditioned drinking response and cerebrum hippocampi cone cell number of the baby rats exposed to the γ rays in different periods of their embryo development. As a result, that pregnant rats exposed to ,137,Cs γ rays in different pregnant periods may induce significant decrease in cerebrum hippocampi cone cell number and achieving rate of conditioned drinking response of the babies. The dose−response relationship can be described by ,Y, = a−b log,10,D,. The achieving rate of conditioned drinking response were significantly correlated to cerebrum hippocampi cone cell number in the babies, and the achieving rate of conditioned drinking response of the babies exposed at pregnant 11d was lower than others exposed at pregnant 16d.