Previously we reported the existence of two distinct cDNAs in rainbow trout that encode for separate preprosomatostatins (PPSS), each containing [Tyr7, Gly10]somatostatin-14. In the present study, we used rainbow trout to further characterize the polygenic origin of somatostatins (SSs), a peptide hormone important in the regulation of growth, development, and metabolism of vertebrates. A two-phase rapid amplification of cDNA ends (RACE)-PCR was used for the isolation of selected cDNAs. We amplified and sequenced a ca. 350-bp 3' RACE-PCR fragment. Based upon this sequence we designed a second gene-specific primer for 5' RACE-PCR which yielded a 452- bp fragment. Sequence analysis revealed a 745-bp cDNA containing the complete 5'-untranslated region, a single initiation site 118 bases from the most 5' end, and a single putative polyadenylation site 17 bases from the most 3' end that was terminated with a polyadenylated tail. The deduced protein is a 114- amino acid PPSS molecule that contains a number of putative processing sites, potentially yielding a 26-amino acid peptide that could be processed further to a 14-amino acid peptide identical in structure to mammalian SS-14. Northern analysis revealed that PPSS-I was expressed in the pancreas, stomach, intestine, and brain of rainbow trout. These results suggest a polygenic origin of SS, possibly resulting from gene duplication events prior to the divergence of teleosts.