Female rats were injected (IP) with either 15, 60, 100, or 150 mg/kg N-ethyl-N-nitrosourea (ENU) on gestation day 18. The neurochemical development of the whole brain and the cerebellum was studied in 21- and 60-day old offspring; pups from mothers subjected to 150 mg/kg were examined only at 21 days. Brain weights were significantly reduced in all ENU-exposed groups at 21 days, and the 60 mg/kg group at 60 days. Whole brain DNA and protein concentrations of most ENU groups were elevated at 21 days and RNA content and RNA/DNA ratios were subnormal. Deficits in whole brain DNA content were observed in all ENU groups on day 60. Cerebellar weight was reduced in the 60 and 150 mg/kg groups on day 21 and in the 100 mg/Kg group on day 60. At 21 days, reductions in cerebellar DNA and RNA concentrations and contents were often recorded in several ENU groups. Cerebellar DNA concentration and content of the 60 and 100 mg/kg groups remained subnormal at 60 days of age. In general, a dose-response effect was evident in the number and magnitude of neurochemical alterations produced by ENU. These data suggest that transplacental exposure to ENU, a potent neurotropic carcinogen, affects the biochemical maturation of the brain.