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Using immunocytochemical techniques the seasonal variation in NPY immunoreactive fibers was investigated in the suprachiasmatic nucleus of both male and female jerboas. During the period of sexual quiescence (autumn), the amount of NPY immunoreactive fibers in the suprachiasmatic nucleus of both male and female jerboas was higher than in the period of sexual activity (spring-middle of summer). Compared with the respective control groups, castration during the period of sexual activity and testosterone or estrogen supplementation in sexually inactive animals did not affect NPY immunolabeling. These results indicate that the seasonal variation observed in NPY immunoreactivity in the suprachiasmatic nucleus of the jerboa is independent of circulating levels of steroid hormones. The possible influence of another hormonal system or a direct influence of an external factor such as photoperiod on NPY content in the suprachiasmatic nucleus remains to be determined.