The role of synthetic seminal plasma peptide, designed using biochemical and mass spectroscopy analyses of native peptides extracted from seminal plasma, was studied in amphibian (Rana esculenta) testicular steroidogenesis. Production of testosterone and prostaglandin F_2α was determined by incubating frog testes with synthetic peptide in vitro. Analysis of the data showed a dose-dependent inhibition of testosterone production (43% at 10^-5 m concentration) without prostaglandin F_2α synthesis being affected. Determination of the peptide activity during the annual R. esculenta reproductive cycle showed inhibition of testosterone production in post-reproductive and recovery periods, suggesting a possible involvement of peptide in gonad steroidogenesis.