In an experiment performed to investigate the effect of LH-RH on the anterior pituitary of rats, we studied the changes in the concentration of 3′,5′-cyclic AMP in the anterior pituitary and the uridine uptake by the anterior pituitary following the intravenous injection of 100 ng LH-RH. The results obtained are summarized as follows: 1) The tissue level of 3′,5′-cyclic AMP in the anterior pituitary reached the highest peak 15 min after the injection of LH-RH. 2) The uridine uptake of the anterior pituitary began to increase 15 min after the LH-RH injection and attained a maximum level (5 to 6 times the normal level) in 30 min. 3) The fact that the uridine uptake was inhibited to a considerable extent by the addition of actinomycin D to the incubation medium suggested that uridine was incorporated into the RNA fraction.