Abstract Pituitary nucleic acid content, [ 3H]-uridine and [ 3H]-leucine incorporation were studied in vitro. Castration caused an increase in the pituitary nucleic acid metabolism. This effect was in vivo reversed by a biphasic action of oestradiol-17β which had an early facilitatory and a late inhibitory influence on the RNA metabolism in the 8th and 16th hours, respectively. To analyse the origin of the biphasic effects of oestrogen, we examined the roles of LH-RH and hysterectomy. LH-RH inhibited the uridine incorporation both in vivo and in vitro by time- and dose-dependent action. Hysterectomy augmented the castrational increase in the RNA metabolism, whereas steroid-free uterine extracts significantly inhibited it. The half suppressive dose (S 50) of the extracts depended on the origin of the uteries extracted. Uterine extracts from animals pretreated with oestradiol were more effective (S 50:0.028 mg protein) than extracts from uteries of intact and castrated animals (S 50:1.18mg and 5.0 mg, respectively). Extracts from cardiac muscle and denatured uterine cytosols have very little effect or none. We concluded that extrapituitary factors and biochemical events may be involved in the oestrogen actions exerted on the pituitary nucleic acid metabolism. A model of the regulation is presented.