In this study, we examined the effect of a putative neurotransmitter or a neuromodulator histamine (HA) on the feeding behavior to elucidate its physiological function in the central nervous system. Rats were implanted with a cannula into the suprachiasmatic nucleus through which HA was continuously infused for 200 hours with an Alzet osmotic minipump. The food intake was recorded automatically. This infusion resulted in decrease in food intake during the dark period and increase in it during the light period which contributed to the decrease in total food intake and increase in the percentage of food intake during the light period. Percentage of food intake during the light period is a good index of the amplitude of the circadian rhythm. Presumably, HA is concerned not only in the meal size, but also in the chronological aspect of the feeding behavior. The administration of H1-antagonist, pyrilamine, antagonized the HA induced increase in food intake during the light period. These findings suggest that continuous infusion of HA affected the feeding behavior which is possibly mediated through the H1-receptors in rat brain.