The concentrations of β-endorphin in discrete regions of the rat brain and pituitary were studied at different ages (8, 24, and 60 days). β-Endorphin concentrations have also been studied 1 month after adrenalectomy, orchiectomy, ovariectomy, thyroidectomy, and/or daily treatment with T4 for 1 month. Moreover, the acute effects of morphine, naloxone, and pentobarbital on brain and pituitary concentrations of β-endorphin have been studied. When necessary, β-endorphin immunoreactivity has been separated from cross-reacting compounds by gel chromatography. The concentrations of β-endorphin in rat brain regions were similar in 8- and 24-day-old rats but increased significantly between days 24 and 60. After adrenalectomy, the concentration of β-endorphin increased significantly in the pituitary, hindbrain, midbrain, and hypothalamus. After thyroidectomy, the concentration of β-endorphin increased significantly in the hypothalamus. A significant decrease in β-endorphin concentration in the pituitary was observed after orchiectomy. Neither ovariectomy nor treatment with T4 induced any change in the concentration of β-endorphin in the brain. A significant increase in β-endorphin concentrations in the hypothalamus and pituitary was observed after pretreatment with the opiate antagonist naloxone or pentobarbital, respectively. No effect of morphine administration on β-endorphin concentration was observed. The results of these studies suggest that brain endorphin concentrations increase with age, and peripheral hormone concentrations can affect the concentrations of β-endorphin in the brain.