A procedure of incubation of thecal layers from rainbow trout (Oncorhynchus mykiss) has been developed, in order to study the in vitro binding of salmon gonadotropin II (sGtH II) during the periovulatory period. The effects of two steroids and growth hormone (GH) on SGtH II binding on thecal layers were investigated in vitro to determine if these hormones are involved in GtH II binding regulation, Steroids alone had no action on GtH II binding, Oestradiol-17 beta with sGtH II had little effect on GtH II binding on thecal layers at the end of vitellogenesis. In contrast, testosterone in combination with GtH II modulated GtH II binding on, thecal layers, being either inhibitory at the end of vitellogenesis, or stimulatory at the peripheral-GV stage, GH, without action alone, induced, in synergy with sGtH II, a significant decrease in GtH II binding on thecal layers at the end of vitellogenesis. Thus, GtH II binding on thecal tissues may be modulated in vitro by steroids, especially testosterone, and by GH, which act through synergism with GtH II.