{Previously, we found that the dose of estradiol (E₂) required to accelerate egg transport increases 5- to 10-fold, in mated compared to cyclic rats. Here we examined protein synthesis in the oviduct of mated and cyclic rats following a single injection of E₂ known to accelerate oviductal egg transport or after concomitant treatment with progesterone (P₄) known to block this acceleration. On Day 1 of the cycle or pregnancy, E₂, P₄, or E₂ P₄ were injected s.c., and 4 h later oviducts were removed and incubated for 8 h in medium with ³⁵S-methionine. Tissue proteins were separated by SDS-PAGE, and protein bands were quantitated by fluorography and densitometry. In mated rats, E₂ and P₄ increased different protein bands and P₄ did not affect the fluorographic pattern induced by E₂. In contrast with mated rats, none of these treatments changed the fluorographic pattern of the oviductal proteins in cyclic rats. Estradiol-induced egg transport acceleration was then compared under conditions in which oviductal protein synthesis was suppressed. Mated and cyclic rats treated with equipotent doses of E₂ for accelerating egg transport also received actinomycin D (Act D) locally. Estradiol-induced oviductal egg loss was partially blocked by Act D in mated but had no effect in cyclic rats. We conclude that the oviduct of mated and cyclic rats differs in that only the former responds with increased protein synthesis to a pulse of exogenous E₂ and P₄ and requires an intact protein synthesis machinery in order to accelerate egg transport in response to E₂.