Two experiments were performed to examine variations in whole-organ concentrations of RNA, DNA and protein, and specific activities of RNA in vivo in the mouse oviduct and uterus during the oestrous cycle and early pregnancy. Concentrations of DNA did not vary in the oviduct at any stage of the reproductive cycle in either study. In the first experiment with Yale Swiss mice, the concentration of RNA in the oviduct was highest at pro-oestrus, somewhat lower at mid-day of oestrus, and lowest on days 2, 4 and 12 after insemination. Concentrations of RNA were essentially identical in the oviducts of inseminated and non-inseminated females at mid-day of the day following ovulation. The second experiment, using CF-1 mice, confirmed these results, and, in addition, examined oviducal and uterine uptake and incorporation of [3H]uridine. Incorporation of the radioactive precursor into RNA was highest in the oviduct at mid-day of pro-oestrus, was markedly decreased before ovulation, and remained relatively low during the period of egg transport in both inseminated and non-inseminated mice. Most of the RNA necessary for appropriate oviducal function during zygote transport, therefore, is synthesized well before ovulation, and relatively little is formed during the 3 days of tubal residence. The general pattern of incorporation of [3H]uridine into RNA in the uterus appeared basically similar to that shown by the oviduct, except at mid-pregnancy when transcriptional activity of the oviduct genome was relatively low. The two organs also differed markedly in the magnitude of the pro-oestrous surge in RNA synthesis and accumulation, with the uterus showing a greater increase.