The duality of salmon gonadotropins has been proved by biochemical, biological, and immunological characterization of two chemically distinc gonadotropins. GTH I and GTH II were equipotent in stimulating estradiol production, whereas GTH II appears to be more potent in stimulating maturational steroid synthesis. The ratio of plasma levels and pituitary contents of GTHs and the secretory control by a GnRH suggest that GTH I is the predominant GTH during vitellogenesis and early stages of spermatogenesis in salmonids, whereas GTH II is predominant at the time of spermiation and ovulation. GTH I and GTH II are found in distinctly separate cells. In trout, GTH I is expressed first in ontogeny, whereas GTH II cells appear coincident with the onset of spermatogenesis and vitellogenesis, and increase dramatically at the time of final reproductive maturation. Comparison of the amino acid sequences of polypeptides and the base sequences of cDNA revealed that salmon GTH I β is more similar to bovine FSHβ than bovine LHβ and salmon GTH II β shows higher homology to bovine LHβ than to bovine FSHβ. The existence of two pituitary gonadotropins in teleosts as well as tetrapods suggests that the divergence of the GTH gene took place earlier than the time of divergence of teleosts from the main line of evolution leading to tetrapods.