1. The effect of a dose of naloxone (1 mg.kg-1 b.w.) on peripheral (plasma, atria) and central (hypothalamus, hypophysis) levels of atrial natriuretic peptide (ANP) was investigated in the rat. 2. In control rats, an acute subcutaneous dose of naloxone produced no significant change in plasma ANP, but a decrease (NS) in atrial ANP concentration. 3. In physically conditioned animals, naloxone produced a significant decrease in atrial ANP levels. Receptor sensitivity may thus be involved in this differential response. 4. In hypothalamus and hypophysis, no effect on ANP concentrations was seen after a high dose of naloxone whether in control or in physically conditioned animals, suggesting peripheral and central ANP might be differently regulated, at least after chronic endurance physical training.