NaDodSO4/PAGE analysis of in vitro translation products of rat submaxillary gland (SMG) mRNAs has revealed an important sexual dimorphism. Moreover, most of the rat male-specific major translation products differ in size from those translated from male mouse SMG mRNAs. To characterize proteins accumulated in the rat SMG under androgen control, a cDNA library was constructed. Here we report the nucleotide sequence of a 0.7-kilobase mRNA that is 1000-3000 times more abundant in male rats than in female rats. The predicted corresponding protein, SMR1, has a molecular weight of 16,000 and contains a signal peptide for secretion and potential signals for glycosylation. An interesting feature of SMR1 is the presence, in a hydrophilic region, of the tetrapeptide Gln-His-Asn-Pro surrounded by two pairs of basic residues that represent potential cleavage sites for maturation enzymes. In rats, the tissue distribution of the SMR1 mRNA is restricted to the SMG and the prostate. Only very low amounts of SMR1 mRNA can be detected in the SMG of male or female mice. Southern blot analysis indicates the presence of three genes in rats but only one in mice. Hypotheses on the physiological role of SMR1-derived peptides in male rats are discussed.