The ultrastructure of the fetal guinea pig neural lobe was studied from day 40 to day 60 of gestation. Pituitaries were taken every five days and at least four glands from each gestational period were examined. Bundles of nerve fibers had invaded the periphery of the gland on day 40. By day 50 axon profiles were distributed throughout the entire posterior pituitary though pituicyte processes continued to act as a barrier between axons and the perivascular space of capillaries. Neural processes established contact with the capillaries between days 55 and 60. Neurosecretory granules (NSG) were present within a few axons on day 40. The number of axons with NSG and the total quantity of granules increased gradually throughout fetal development. Electron-lucent granules (microvesicles) were observed infrequently until the day of birth. A population of dense-cored vesicles, 70-80 mmu in diameter, was present from day 50 onward; a second population with larger diameters was also present throughout the developmental sequence and these increased from 90-130 mmu in diameter to 170-220 mmu in diameter between days 40 to 60. The presence of neurosecretory granules is discussed in relation to the onset of synthesis and storage of neurohypophysial hormones.