The experiments of mechanical isolation of medial hypothalamus from the lateral hypothalamus and the preoptic anterior hypothalamic (POAH) region in rats showed that: 1. The interruption of neural connections between POAH area and medial hypothalamus do not prevent the decrease of food intake which normally occur in a hot environment. 2. At 33 degrees C, hyperphagic rats gained more weight than sham-operated ones. 3. At 4 degrees C, rats made hyperphagic by hypothalamic isolation do not ajust their food intake for a long period and do not gain weight. 4. The excitatory pathways of the feeding center from the POAH area do not penetrate directly into the lateral hypothalamus, but rather into the medial retrochiasmatic area. 5. The temperature influences the diurnal pattern of feeding only in rats with intact or unilateral neural connections of the hypothalamic structures 6. It seems that the thermostatic mechanism, which is a potent regulator of feeding, is closely associated with the central control of thyrotropin release, and that the hypothalamic structures may be considered only as a necessary link in the nervous mechanism involved in feeding control.