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The morphological effects of colchicine on the entire neurosecretory (NS) tract and on various hypothalamic nuclei have been studied. The perturbations in axonal flow, indicated by the accumulation of NS material, coincide with fragmentation of the cytoplasmic membranes, i.e. the Golgi apparatus and the endoplasmic reticulum, whereas the neurotubules remain relatively well preserved. Autophagic destruction of NS material is observed along the entire length of the NS fibres. The rapid and systematic changes in the axoplasmic reticulum, known to store calcium, lead us to envisage a role for this system-similar to that of the sarcoplasmic reticulum-in controlling the transport of NS vesicles. The junctional zone between the stalk and the neural lobe seems to play a particular role in the transport of NS material to the posthypophysial terminals of the NS axons. Colchicine provokes an increase in dense-cored vesicles in most of the neurons of the other hypothalamic nuclei studied: arcuate, suprachiasmatic, periventricular and ventromedial. Membranous alterations are also observed in these sites. Colchicine administered to animals which were hypothyroid, castrated or adrenalectomized, reveals stimulated neurons, identified by their excessive content of dense-cored vesicles. These neurons display no specific localization, for they occur in all hypothalamic nuclei, irrespective of the stimulation. The frequency of stimulation of neurons of the periventricular nucleus is striking.