Argininosuccinate lyase (ASL), the fourth enzyme of the urea cycle, belongs to a group of liver enzymes appearing in the late foetal period in the rat. Several hormones, including glucocorticosteroids and insulin have been implicated in the control of the development of this enzyme activity. In this study, the cloned cDNA was used to measure the relative abundance of ASL mRNA in the livers of rats at various stages of perinatal development and in cultured foetal hepatocytes during hormonal manipulations. The ASL mRNA was first detectable on day 15.5 of gestation and increased in amount concomitantly with the rise in the enzyme activity, suggesting that the appearance of enzyme activity reflects the turning on of specific gene transcription. When foetal hepatocytes were exposed to dexamethasone, an increase in ASL mRNA was detected, which was completely abolished by addition of actinomycin D, suggesting a transcriptional effect of the steroid. In contrast, administration of cortisol to foetuses in utero had no effect on the mRNA level, suggesting that the steroid action is inhibited in the intra-uterine environment. Insulin might be the inhibiting factor since it completely repressed the dexamethasone-induced accumulation of ASL mRNA in foetal hepatocytes. These data were confirmed in vivo by experiments using streptozotocin, which produces insulin-depleted foetuses and causes the accumulation of ASL mRNA. This regulation of ASL mRNA by glucocorticoids and insulin could account for the modulation of the enzyme activity observed in vivo and in vitro.