The microvascular pressures that perfuse the anterior pituitary gland with blood were not known. We now report the direct measurement of these pressures in the urethane-anesthetized rat. The infundibular stalk and ventral surface of the anterior pituitary gland were surgically exposed via a parapharyngeal approach and a micropressure transducer inserted into the lumen of hypophysial portal vessels under direct microscopic observation. A Weiderhielm-type servo-controlled pressure system was used to record the pressures. Continuous pressure recordings up to 30 min in duration were made in long hypophysial portal vessels ranging in diameter from 10 to 50 µm in adult, female Sprague-Dawley rats. The mean pressure recorded from these vessels was 4.0 cm H2O (2.7 mm Hg.) A small increase in systemic pressure produced by a rapid saline infusion into a cannulated femoral vein resulted in a mirrored but much greater magnitude increase in pressure to the hypophysial portal vessels. This finding suggests that pressure within the portal vessels is in some instances closely coupled to systemic blood pressure. The low pressures recorded in hypophysial portal vessels correlate well with pressures measured in the hepatic portal vasculature. The porosity of fenestrated capillaries surrounding anterior pituitary cells is hemodynamically essential, since the low hydrostatic pressures alone would be inappropriate for rapid and thorough exchange