The developmental interrelationship between the neurosecretory pathway and the hypophysial portal system has been lightmicroscopically studied in young rats. On the first day after birth, AF-positive neurosecretory substance appeared in the supraoptic nucleus, but not in the paraventricular nucleus, where it appeared four days later. On the 4th day of postnatal life, the primary plexus of the hypophysial portal vessels began to penetrate the external layer of the median eminence from the supratuberal plexus. At the same time, few delicate fibers containing AF-positive materials were detected in the intermediate layer of the median eminence, but not in the posterior lobe. Occasionally they deviated from the main route, i. e., the hypothalamo-hypophysial tract in the intermediate layer, and entered the external layer. On the other hand, nerve fibers demonstrated by silver impregnation (Palmgren, 1948) were traced from the intermediate layer into the posterior lobe via the pituitary stalk. Several days later, the capillary loops penetrated deeply into the external layer of the median eminence. Some of them reached the intermediate layer and established contact with the AF-positive neurosecretory fibers. Subsequently, the AF-positive neurosecretory fibers increased in number within the intermediate layer of the median eminence, and primary capillary loops of the portal vessels developed into their adult form. At days 9 and 10 after birth AFpositive neurosecretory fibers appeared within the posterior lobe.