Autoradiography was applied to the study of the effect of extirpation of the epiphysis in rats on the level of H3-leucine incorporation by the neurons of 8 couples of the nuclei localized in the anterior, middle and posterior portions of the hypothalamus. Experiments were conducted on female animals of the prepubertal and reproductive age. Pinealectomy was combined with binding (a factor intensifying the physiological status of the epiphysis) and continuous illumination (a factor inhibiting the function of this gland). In the blinded prepubertal rats pinealectomy enhanced the H3-leucine incorporation into the supraoptic nucleus by 38%, into the paraventricular--by 26%, into the dorsomedial--by 61%, into the premammillary dorsal--by 58%, into the ventro-medial--by 40%, into the premammillary ventral--by 36%, and into the anterior hypothalamic--by 26%. In mature rats this index exceeded, after pinealectomy, such in the sham-operated animals (control) in the supraoptic nucleus by 48%, in the paraventral--by 40%, in the premamillary ventral--by 80%, in the arcuate--by 48%, in the premammillary dorsal--by 35%, in the ventro-medial--by 29%, and in the dorso-medial--by 26%. Continuous illumination reduced the stimulating influence of pinealectomy on the H3-leucine incorporation into the neurons of the premammillary and the arcuate nuclei, without changing the difference between the experiment and control in the macrocellular nuclei. On the whole, the results obtained permit a conclusion to be drawn on tonic inhibition by the epiphysis of the extensive hypothalamic zones.