The significant relationship of drinking to brain opioid receptors was determined using naloxone, a preferential antagonist of opioid µ receptor, in water-deprived rats. Whereas 48-hour food deprivation did not affect naloxone binding in any brain regions, 48-hour water deprivation caused a significant decrease in naloxone binding in the striatum, but not in the other brain regions. The striatal naloxone binding was found to progressively decrease with a period (0–48 h) of water deprivation. No significant change in naloxone binding in the hypothalamus or midbrain was observed during water deprivation. Water deprivation did not change the affinity constant of naloxone binding in the striatum. These observations indicate that the change in the striatal opioid receptors was specifically associated with thirsty sensation in the r