The effect of crowding on the pituitary-gonadal axis was studied in adult male rats. Crowding reduced body weight gain, did not alter relative adrenal weight and increased relative testis weight. Neither basal levels of corticosterone nor its response to acute stress were altered by crowding. Likewise, LH secretion was similar in control and crowded rats. Prolactin levels tended to be lower in crowded than control rats, but they did not reach statistical significance. In contrast, impaired testosterone secretion was observed in crowded rats, at least in part due to reduced Leydig cell responsiveness to gonadotropin release. In addition, some mechanisms other than pituitary-adrenal hyperactivity might be responsible for reduced testosterone secretion during crowding. © 1986 Informa UK Ltd All rights reserved: reproduction in whole or part not permitted.