To analyze the potential action mechanism of endogenous steroids in spermatogenesis, changes in gene expression were measured during testicular maturation and germ cell development using homologous cDNA microarray, in relation to blood steroid concentrations. We also investigated the effect of testosterone (T), 11-ketotestosterone (11KT), 11beta-hydroxy-androstenedione (11OH-And), estradiol (E2) and of the meiosis inducing progestin 17,20beta-dihydroxy-4-pregnen-3-one (MIS or 17,20 DHP) on testicular gene expression at the beginning of spermatogenetic development in rainbow trout. Numerous transcription factors significant to gonad development (Tab. II) were found to be modulated after steroid treatments. A large majority of the genes up or down regulated following exogenous steroid treatments demonstrated a specific expression pattern during the spermatogenetic development of the testis. Furthermore their pattern of expression often appeared related to the natural changes in sex steroid circulating levels.