Isolated trout steroidogenic testicular cells were cultured for 10-15 days, either mixed with other round cells or after enrichment in interstitial cells. Free and conjugated progestagen and androgen secretions were assayed using specific radioimmunoassays (RIA). Free progesterone, 17 alpha-hydroxyprogesterone (17 alpha-OH-P), 17 alpha-hydroxy,20 beta-dihydroprogesterone (17 alpha,20 beta-OH-P), androstenedione, testosterone (T), and 11-ketotestosterone (11KT) were produced by testicular cells prepared from testes in spermatogenesis and mature testes. Discrete amounts of dehydroepiandrosterone (DHA) and of estradiol were secreted by mixed testicular cells prepared from mature testes, but no estradiol was detected in interstitial cell media. Conjugated androgens were produced by interstitial cells. While the production of progestagens by cells from spermatogenetic and mature testes either remained constant or increased throughout culture duration, those of free and conjugated androgens progressively decreased to low values whatever the components added to the medium. When salmon gonadotropin (s-GtH) was present permanently, androgen (free and conjugated) and progestagen secretions were stimulated for 3 to 4 days. When GtH was present discontinuously (1 day in every 3 days), the sensitivity of the cells was maintained for at least 7 days. While the GtH-stimulated/basal ratio was high for androgens, it was rather low for 17 alpha 20 beta-OH-P as compared to the values obtained with testis fragments. Trout serum (5%) stimulated the secretion of free and conjugated T and 11KT when testes were mature, but not when they were in spermatogenesis, while it stimulated 17 alpha 20 beta-OH-P secretion at the two stages. Total trout lipoproteins (125-500 micrograms/ml) stimulated 17 alpha 20 beta-OH-P secretion by cells from spermatogenetic testes, but not 11KT secretion.