Intersexual differences and annual cytological changes in presumptive gonadotropin-producing cells (gonadotropes) were examined immunohistochemically in mid-sagittal sections of the pituitary gland of adult Hynobius nigrescens by the use of a monoclonal antibody against bullfrog luteinizing hormone beta-subunit (LH beta). Total immunoreactive areas for LH beta and profiles of the pars distalis in the mid-sagittal plane of the gland were estimated with an image analyzer. Gonadotropes consistently were distributed in the pars distalis near the median eminence and the pars intermedia in both sexes throughout the year; an intersexual difference was found in their distribution pattern. Gonadotrope cells other than these consistent ones distributed in the peripheral region of the pars distalis frequently were observed in females, but not in males. The proportion of gonadotropes among all pars distalis cells, as well as the mean area and number of gonadotropes, was significantly greater in females than in males throughout the year. In males that had a typical aquatic-phase morph and actively exhibited reproductive behavior, both area and number of gonadotropes, as well as profile estimates of pars distalis size, were the greatest of all monthly values. In females, gonadotrope cells increased in area and number from July-January during the terrestrial phase of the nonbreeding season. This increase was concurrent with the development of ovaries. This consecutive increase and development may explain the absence of a second oogenic phase during early spring.