Some aspects of uterine RNA synthesis including [3H]-uridine incorpo ration into RNA, activities of RNA polymerases and ribonucleases were studied. It was observed that both normal and pregnant animals, kept on protein-free diet for 15 and 20 days, showed a significant increase inin vivo uptake of [3H]-uridine into total RNA. Activities of RNA polymerase I and polymerase III increased two-fold in animals kept on a protein-free diet; however, RNA polymerase II activity was unaffected by protein restriction. In animals kept on protein-free diet where pregnancy was maintained by exogenous estrogen and progesterone, specific activity of nuclear RNA was further increased and the activities of RNA polymerases I, II and III markedly increased. Levels of RNase were also increased significantly during protein deficiency, thus showing a rapid turn-over of uterine RNA. These observations indicate that during protein restriction, uterine RNA synthesis is regulated at transcriptional level by a selective stimulation of RNA polymerase and RNase also plays an important role.