Intrauterine growth retardation was induced in the rat by clamping the uterine artery on day 17 of gestation. The effect of hypotrophy on DNA synthesis was studied in two different cerebral structures: hippocampus and cerebellum. Accumulation of DNA in these structures was biochemically measured in parallel to the incorporation of methyl-[14C]thymidine into nucleic acid at different ages and correlated with autoradiography. The various metabolites of thymidine in acid-soluble fraction were determined by using chromatographic procedures. Phosphorylation defects or reduced utilization of thymidine were found in hypotrophic rats and may delay the DNA synthesis. An essay of catch-up occurred with a different timing according to the cerebral region studied. A morphological and DNA synthesis. An essay of catch-up occurred with a different timing according to the cerebral region studied. A morphological and autoradiographic study after incorporation of [3H]thymidine was carried out in parallel. The neuronal and glial components of cytogenesis were analyzed separately and a good correlation was observed between histological and biochemical data in both groups of animals.