Histaminergic neurones may be involved in the regulation of feed intake. Since TCDD causes anorexia in rats, histamine concentrations were measured in several hypothalamic nuclei involved in feeding regulation. Histamine concentrations were not changed in medial or lateral accumbens, suprachiasmatic, paraventricular, arcuate, ventromedial, dorsomedial or perifornical nuclei, or in lateral hypothalamic area, cortex, or pineal gland. There was, however, an increase in histamine concentration in median eminence 25 h after the administration of TCDD.