The human mas oncogene was originally detected by its ability to transform NIH 3T3 cells. We previously showed that the protein encoded by this gene is unique among cellular oncogene products in that it has seven hydrophobic potential transmembrane domains and shares strong sequence similarity with a family of hormone-receptor proteins. We have now cloned the rat homolog of the mas oncogene, determined its DNA sequence, and examined its expression in various rat tissues. A comparison of the predicted sequences of the rat and human mas proteins shows that they are highly conserved, except in their hydrophilic amino-terminal domains. Our examination of the expression of mas, determined by RNA-protection studies, indicates that high levels of mas RNA transcripts are present in the hippocampus and cerebral cortex of the brain, but not in other neural regions or in other tissues. This pattern of expression and the similarity of mas protein to known receptor proteins suggest that mas encodes a receptor that is involved in the normal neurophysiology and/or development of specific neural tissues.