Embryonic cortical neurons in culture contain transmembrane amyloid precursor protein (APP) capable of associating with the detergent- insoluble cytoskeleton through interactions requiring the presence of its C-terminal. These transmembrane APPs are not detectable at the surface of living cells. When neurons are fixed with paraformaldehyde alone, APP is mainly visualized close to the membrane of the axon and cell body of 40% of neurons, with virtually no dendritic staining. Membrane permeabilization with detergent or methanol extends APP immunostaining to 100% of the cells and to all compartments, including the dendrites. Taken together, these results suggest that APP in embryonic neurons is present in two compartments, one more readily detectable in some axons and cell bodies and the other distributed throughout all neurons. The axonal and somatic pool of APP detectable after paraformaldehyde fixation alone is highly and rapidly augmented after exposure to calcium ionophores. We propose that calcium entry increases the amount of axonal APP close to the cell surface, but that the stabilization of the protein at the cell surface and its subsequent secretion require further physiological stimuli.