Exocytosis is clearly shown in freeze-fracture preparations to be the mechanism for neurosecretion granule release from axon endings in the crayfish sinus gland. The cytoplasmic leaflet (A-face) of axon ending membrane is characterized by randomly situated depressions representing invaginations of the axolemma, which are in contact with limiting membranes of neurohormone granules in the subjacent cytoplasm. The extracellular leaflet (B-face) of the axolemma at release sites exhibits complementary volcano-shaped protrusions which are cross-fractures through necks of channels formed by invaginating plasma membrane in contact with underlying neurosecretion granules. Structural variation in B-face protrusions is consistent with a spectrum of exocytotic profiles in various stages of formation, and with granules at different stages of passage out of the endings. Evidence in this study suggests that formation of exocytotic structures may begin by alteration of axon membrane structure at the neurosecretory ending-hemolymph interface prior to contact of the neurohormone granules with the axolemma. Limiting membranes of neurosecretory granules exhibit protrusions which appear to interconnect granules adjacent to release sites and to attach granules to the axolemma. Freeze-fracture is clearly shown to be an invaluable tool for monitoring the degree of exocytosis exhibited by sinus glands under normal conditions and under experimental acceleration of hormone release. This technique is capable therefore, of detecting slight increases in numbers of exocytotic profiles much more quickly and accurately than the examination of random thin sections.