The dependency of the oestrogen-induced increase in uterine cGMP content towards the cytosol-nuclear receptor system was investigated. The following observations were made: (1) With oestradiol-17 beta (E2-17 beta), U11-100A (UA) or CI-628 (CI) the cGMP response elicited in the uterus of immature rats followed a course that was parallel to (yet delayed by about 1 h from it) the known time-course evolution of nuclear occupancy by the complex formed by each compound with the oestrogen-receptor. (2) While a marked (about 2-fold) increase in uterine cGMP content was obtained with 0.1 microgram E2-17 beta, oestradiol-17 alpha (E2-17 alpha) given at the same dose had no effect on uterine cGMP. (3) The 2--3 h response to E2-17 beta (or to UA) could not be obtained in animals that had received a first injection of E2-17 beta, 2 h, or of one of the anti-oestrogens UA or tamoxifen, 20--22 h prior to the test injection of E2-17 beta. Those 3 treatments have in common that, at the time indicated, they create a state of depletion in the uterine cytosolic receptor population. The cGMP response to E2-17 beta was restored 20--22 h following a first injection of E2-17 beta. This time is known, in this case, to correspond to full replenishment of the cytosol-receptor population. In all those tests, the wet weight increase, measured in the same organs, behaves exactly as did the cGMP response. These results support the conclusion that the increase in uterine cGMP after oestrogen administration to the immature rat, represents a true hormonal action which, like other uterotrophic actions of oestrogens, involves binding of the hormone by the cytosol receptor.