Somatic cells (interstitial cells and Sertoli cells) were prepared either as single cells or in clusters, from spermatogenic and mature trout testes, according to Loir (1988), and cultured for 10-14 days. Sertoli cells are 3 beta-HSD negative when prepared from testes resuming spermatogenesis and from mature testes, but they are 3 beta-HSD positive in spermatogenic testes. Progesterone, 17 alpha-hydroxyprogesterone (17 alpha-OH-P), and free androgens are secreted by interstitial cells, 11-ketotestosterone (11KT) being the predominating steroid produced immediately after seeding. These cells also produce high levels of glucuronated androgens. At least in mature spermiating testes they do not secrete estradiol. After isolation, interstitial cells would lose most of their ability to secrete 17 alpha-hydroxy,20 beta-dihydroprogesterone (17 alpha 20 beta-OH-P) but they would recover it later. Testicular spermatozoa, which convert 17 alpha-OH-P independently of s-GtH, constitute a second source of this progestagen. In addition, our results suggest that Sertoli cells could be able to secrete 17 alpha-OH-P and also progesterone. A possible participation of the intralobular production of the former progestagen to the local regulation of germ cell maturation is evoked.