The developmental ratardation of brain in rats induced by prental exposure to HTO had observed by measuring the contents of cerebral amino acids. The HTO was injected intraperitoneally at the 11 th day of gestation with concentration of 7.4×10,3,—3.7×10,6, Bq/ml (body water). At 18-day-old the young rats were killed and their brains were saparated from skulls. The brains were used to make biochemical speciments for measuring the contents of amino acids.The results showed that the contents of amino acids increases with the dose increased and the relationship between the precentage of the increased contents of amino acids in brain and logarithm of absorbed doses (D, Gy) give a good fit to liner regression equation ,http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=42650457&type=,http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=42650489&type=,http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=42650488&type=, in range of the absorbed doses from 0.0038—1.9 Gy.