Specific binding [3H]naloxone to midbrain and forebrain was examined from chicks which were either 1, 7, 14, 21 or 28 posthatch days of age. While there was a significant age-related increase in the total number of [3H]naloxone binding sites in both brain regions, the increase was greater in magnitude in forebrain than in midbrain. In both brain regions, there was a concomitant age-related decrease in the density of receptor sites without any alteration in receptor affinity for naloxone. The decrease in receptor density was more rapid in onset and was greater in magnitude in midbrain than in forebrain.