Previous studies from our laboratory have shown that salmon gonadotropins GTH I and GTH II have similar effects on testicular steroidogenesis in vitro. To determine whether the relative potencies of GTH I and GTH II changed during late stages of spermatogenesis, the effects of GTH I and GTH II on in vitro steroid production by testicular tissue from fish in stages IV to V of spermatogenesis were examined. Fragments of testicular tissue were incubated with GTH I and GTH II for 18 h at 15 degrees C. The in vitro production of 11-ketotestosterone (11-KT), testosterone (T), 17 alpha-hydroxyprogesterone (17OH-P), and 17 alpha,20 beta-dihydroxy-4-pregnen-3-one (17,20 beta-P) was determined by RIA. GTH I and GTH II were equipotent in stimulating the production of 11-KT, T, and 17,20 beta-P (17OH-P was not detectable) by testicular tissue in stage IV of spermatogenesis. The sensitivity of testicular tissue to the steroidogenic effects of GTH II increased as spermatogenesis progressed. In contrast, the sensitivity of testicular tissue to the effects of GTH I on 17,20 beta-P production declined from stages IV to V, while the sensitivity of testicular tissue to the effects of GTH I on 11-KT production remained unchanged. These changes in sensitivity to GTH I and II are consistent with previously reported changes in GTH receptors in salmon testis and support the hypothesis that GTH I and GTH II may have different roles during spermatogenesis.