1. Techniques have been developed to measure the concentrations of the ribonucleotides of the immature rat uterus in vivo. Tissue was frozen rapidly in liquid nitrogen, ground to a fine powder, dispersed in frozen perchloric acid and thawed slowly. Nucleotides were separated from other acid-soluble constituents on short columns of polyethyleneimine-cellulose and the mixture was resolved into individual nucleotides by two-dimensional thin-layer chromatography on polyethyl-eneimine-cellulose plates. 2. The nucleotides of immature rat uterus consisted of approximately 75% of ATP-ADP, 10-12% each of GTP-GDP and UTP-UDP and less than 2% of CTP. 3. Injection of oestradiol (5mug) promoted a linear decrease in the amounts of purine nucleotides to approximately 60% of control values in 4-5h, followed by a return to greater than control values in 8-10h. Concentrations of the pyrimidine nucleotides remained constant for 4-6h and then increased to 200% of control at 12h after hormone treatment.