Tissue distribution and intracellular localization of dynamin by immunoblotting and immunocytochemistry is investigated in this study. Dynamin was widely expressed in all the neurons we examined, and was especially abundant in the central nervous system after maturation, although its expression presented regional heterogeneity. Dynamin was present most abundantly in cerebellar Purkinje cells and hippocampal pyramidal cells, and to a lesser extent in motor neurons and peripheral nerves. However, dynamin was nearly absent in cells such as anterior pituitary cells and adrenal medullary cells which secrete mainly dense cored vesicles. Dynamin was localized not only in cell bodies, axons, and synapses but also in dendrites. Subcellular fractionation indicated that dynamin existed in the membrane fraction as well as in the soluble fraction. In ligated peripheral nerves, dynamin colocalized with tubulovesicular membranous organelles transported mainly anterogradely. By transfection of dynamin cDNA into mouse fibroblast L-cells, we showed it colocalized with some membranous organelles but not with microtubules. Our results show that dynamin is associated with membranous organelles in vivo, although a certain amount of dynamin also exists in the soluble fraction and is distributed diffusely throughout mature neurons. The data suggest that dynamin's fundamental role involves membrane trafficking in neurons in the central nervous system rather than in sliding microtubules as a motor protein.