The expression of two genes, coding for argininosuccinate synthetase (ASS) and argininosuccinate lyase (ASL), enzymes which synthesize arginine, was studied by Northern analysis in various tissues of fetal rats. The highest expression of both genes was seen in the small intestine, liver and kidney of the fetus. The developmental expression was observed in the kidney where both mRNA levels remained low during the fetal period and they increased concomitantly in both kidney and liver throughout the perinatal life, suggesting that the aptitude to synthesize arginine appeared and developed within the same period in both organs. This developmental activation of the ASS and ASL genes expression corresponded, at least in part, to a transcriptional mechanism in both tissues, as measured by run-on assay. Bilateral adrenalectomy showed that glucocorticoids did not appear to control the developmental expression of both genes in the kidney, in contrast to the situation observed in the liver.