The purpose of this investigation was to study the effects of various hormones on the surface morphology of 20-day-old rat testicular cells in culture. Aggregates containing primarily Sertoli cells and germinal cells were obtained by enzymatic digestion. The surface morphology of the cells composing these aggregates was characterized under various culture conditions using light microscopy and scanning electron microscopy. The cytoplasmic processes of Sertoli cells became highly branched and filamentous after being cultured in the presence of rat, human or ovine FSH. Identical branching and filamentation was observed when Sertoli cells were cultured in rat TSH. Finally, numerous large blebs were observed on the surfaces of germinal cells cultured in the presence of insulin. These results suggest that the branching and filamentation of Sertoli cell cytoplasm observed after FSH stimulation are not specific for that hormone.