We report the primary structures of two mRNA species (GH1 and GH2), each predicted from the cloned cDNA and genomic gene sequences, that encode growth hormone in rainbow trout (Salmo gairdneri). Both GH1 and GH2 mRNA contain open reading frames comprising 630 nucleotides and encode 210 amino acid residues, of which 11 are variant. The translated regions of mRNA are flanked by a short 5'-untranslated sequence, which is highly conserved, and a relatively long 3'-untranslated sequence, which is highly divergent. The differences at the 3'-untranslated regions suggest that the GH1 and GH2 mRNA originate from different loci. RNA blot analysis of trout pituitary RNA using an oligonucleotide probe specific for the GH2 sequence indicates that the cloned gene is expressed. The GH1 and GH2 mRNA likely are transcribed from two distinct loci, which were duplicated during tetraploidization of the salmonid genome between 50 and 100 million years ago.