In order to determine whether or not ovum transport acceleration induced by estradiol (E2) requires RNA and protein synthesis in the oviduct, inhibitors of RNA and protein synthesis were injected locally in rats treated with E2. We also tested whether administration of oviductal RNA from E2-treated rats could mimic the effect of E2 on ovum transport. Rats on Day 2 of pregnancy were given a single s.c. injection of 10 microg E2 and an intraoviductal (i.o.) injection of actinomycin D, alpha-amanitin, or cycloheximide (Chx). In control groups, either the steroid or the inhibitor or both were replaced by the respective vehicle. RNA obtained from oviduct or ileum of E2-treated rats or from the oviduct of propylene glycol-treated rats was injected into the oviducts of recipient rats on Day 1 of pregnancy. Animals were autopsied 24 h later to determine the number and distribution of eggs in the genital tract. All three inhibitors partially blocked the E2-induced ovum transport acceleration, whereas administration of inhibitors alone did not affect oviductal egg recovery. Only oviductal RNA obtained from E2-treated rats decreased the number of oviductal eggs (active extract). To interpret this finding, the active extract was preincubated with RNase or DNase before i.o. administration. Other groups of recipient rats also treated with active extract were injected s.c. with Chx, or their uterine horns were ligated to disclose the fate of the missing oviductal eggs. Active extract treated with RNase did not decrease the number of oviductal eggs; Chx blocked the effect of the active extract; and eggs missing from the oviduct were partially recovered in the uteri of ligated recipient rats. It is concluded that protein synthesis in the oviduct is required for the full effect of E2 on ovum transport and that one or more RNA species induced by E2 in the oviduct are by themselves able to mimic, and therefore mediate, the effect of E2 on ovum transport.