1. The effect of 10 day treatment with growth hormone (GH) (1 mg/kg body weight/day) and somatostatin (SRIF) (0.25 mg/kg body weight/day) subcutaneously on the activity of 5-HT1 receptors in rat hypothalamic, pituitary and cerebral cortical membrane fractions was studied using [3H]5-HT as radioligand. 2. The administration of GH and SRIF significantly decreased the 5-HT1 binding capacity and affinity in the hypothalamus. 3. In the pituitary the 5-HT1 receptor activity was also significantly decreased after both hormonal applications. 4. In the cerebral cortex the 5-HT1 receptor affinity was significantly decreased and the binding capacity was increased. 5. The results obtained indicate that GH and SRIF decrease 5-HT1 receptor activity. 5-HT1 receptors are possibly involved in the 5-HT controlled GH feedback autoregulation mediated by SRIF.