[3H]Leucine-labelled haemoglobin was prepared from rat reticulocytes incubated in the presence of [3H]leucine. Conceptuses from 9-5-day pregnant rats were incubated in vitro tox 48 h, with [3H]leucine-labelled haemoglobin present for the final 12, 8, 4, 2 or 0.5 hours. Radioactivity accumulated in visceral yolk sac and in embryonic tissue. When exposure to labelled haemoglobin was for only a short period before harvesting, all the radioactivity found in the embryo and most of that found in the visceral yolk sac trichloroacetic acid-soluble (i.e. associated with free amino acid rather than with protein). After longer exposures the proportion of radioactivity that was acid-soluble decreased to minimum values of about 20%. SDS-polyacrylamide gel electrophoresis of the protein-associated radioactivity in visceral yolk sac and embryo was performed. After exposure to labelled haemoglobin for 1 h only prior to harvesting, the yolk sac contained a single peak of radioactivity coincident in mobility with haemoglobin. The embryo contained no protein-associated radioactivity. After exposure to labelled haemoglobin for 12h, many protein bands in both yolk sac and embryo were radiolabelled. Thus a single radiolabelled protein pinocytically captured by the visceral yolk sac can give rise to the presence of many labelled proteins in embryo and visceral yolk sac. These results indicate that the source protein underwent proteolytic digestion and that the amino acids generated were re-utilized for protein synthesis in both embryonic and visceral yolk-sac cells.