Objective: Evidences implicate roles for prolactin (PRL) in the regulation of embryonic growth. To clarify the roles of PRL in rat embryogenesis we examined the uptake and expression of the hormone in embryonic tissues. Methods: Nine and a half day postimplantation rat embryos were cultured in vitro for 44h in rat serum and serum depleted of low molecular weight molecules (retenate). The embryos were transferred to M199 for the last 4h, and 12.8 ng/ml rat PRL was added to culture medium for different times (4h - 15 min) and/or different temperatures (37ºC and 4ºC). As a control tissue, pituitary glands from 11.5 and 18.5d pregnant rats were used. Embryos and tissues were then examined by an indirect immunofluorescence protocol. Results: The pituitary glands showed positive immunoreactivity for anti-PRL antibody whilst there was no stain in the control brain tissue. Immunoreactivity was observed in embryos grown in rat serum, and intensity was much greater in the presence of additional rat PRL, whilst there was no immunoreactivity detected in those grown in retenate only. Shorter incubations and incubations at 37ºC caused a greater immunoreactivity for PRL, suggesting that this is an active and temperature dependent metabolic process. Conclusion: These results show the uptake and distribution of PRL by the yolk sac and embryonic tissues which might be interpreted for the presence of PRL receptors.