The response of neurones in the medial basal hypothalamus to electrical stimulation in the suprachiasmatic/preoptic region has been studied in rats anaesthetised with urethane. Neurones in the hypothalamic arcuate, periventricular, ventromedial and dorsal premammillary nuclei were antidromically activated by the stimulation showing that such cells send processes in rostral direction outside the median eminence. Tuberoinfundibular and rostrally projecting units were found interspersed in the "hypophysiotrophic" area.