The Mas protooncogene on mouse chromosome 17 encodes a mitogenic G-protein-coupled cell surface receptor. We investigated the allele-specific expression pattern of the Mas gene on the basis of its proximity to the known imprinted gene for the insulin growth factor type II receptor (Igf2r). Phenotyping of mRNA demonstrated exclusive expression from the paternal allele in all embryonic tissues, including visceral yolk sac, between 11 and 12.5 days of gestation. By 13.5 days of gestation the paternal allele-specific expression of Mas was restricted to heart, tongue and visceral yolk sac, whereas all other tissues exhibited relaxation of the parental imprint. Our results demonstrate parental imprinting of Mas and suggest that the maternally inherited allele is transcriptionally repressed in a developmental and tissue-specific manner.