Methods for the measurement of nuclear receptors for oestradiol (E2) and progesterone (P) in mouse uterus and sheep endometrium have been established. Scatchard analysis of nuclear receptors gave the following dissociation constants (nM) and binding site concentrations (pmol steroid/mg DNA): E2 receptor, mouse 3.76 and 2.68, sheep 2.03 and 5.37; P receptor, mouse 5.77 and 2.52, sheep 5.34 and 3.58. The effects of a single injection of P on nuclear and cytosol levels of E2 and P receptors have been contrasted in these tissues from E2-treated mice and sheep. In both species, P treatment resulted, in 30-120 min, in depletion of its own cytosol receptor and accumulation of its nuclear receptor. A significant reduction of cytosol E2 receptor was seen only in the mouse at 24 h. P decreased the nuclear E2-receptor level in the mouse at 2-8 h, but had no such effect in the sheep. The results indicate that the anti-uterotrophic action of P in mouse uterus is caused by an early direct effect of P on nuclear E2-receptor retention, and appear also to explain why P is not anti-uterotrophic in sheep uterus.