Human fetuses, 11.5 to 16.8 weeks' gestational age, were perfused with silicone rubber compound to study the hypothalamo-hypophyseal portal system. Impregnated vessels were observed throughout the median eminence, infundibular stalk, pars tuberalis and both lobes of the pituitary gland in all specimens. The data suggest that an intact portal system for the transport of hypothalamic releasing factors to the pituitary gland is established as early as 11.5 weeks' gestation.