Abnormal metabolic processing of the beta/A4 amyloid precursor protein (APP) has been implicated in the pathogenesis of Alzheimer disease. Several aspects of normal APP processing have been elucidated, but the precise cellular trafficking of APP remains unclear. To investigate APP trafficking pathways further, we have examined the subcellular distribution of APP in rat brain tissue and a variety of cultured cell types, and correlated this distribution with the biochemical processing of APP. In immunofluorescence microscopy of rat brain sections, APP immunoreactivity was concentrated in the Golgi complex and in proximal axon segments. In addition, a lower level of punctate fluorescence was visible throughout the neuropil. By immunoelectron microscopy of rat brain tissue fragments, APP was found associated with Golgi elements and with medium-sized, invaginated vesicles in both axons and dendrites. Prominent localization of APP to the Golgi complex was also found in primary cultures of rat hippocampal neurons and in non- neuronal cell lines. When cultured cells were treated with brefeldin A (BFA), APP immunoreactivity changed from a Golgi-like to an endoplasmic reticulum-like distribution. No APP was detected in the BFA-induced reticulum identified by the transferrin receptor, indicating that concentration of APP in the Golgi does not reflect recycling between the trans-Golgi network and early endosomal system. In immunoblots of BFA-treated cells, there was an accumulation of full-length APP and inhibition of APP secretory processing. Treatment with phorbol ester resulted in a marked elevation of APP secretion, but no obvious redistribution of APP immunoreactivity was apparent at the light microscope level. The lysosomotropic drug chloroquine induced accumulation of APP in cell lysates, as seen by immunoblotting. Immunofluorescence microscopy of chloroquine-treated cells demonstrated a colocalization of APP with the lysosomal marker Igp 120, whereas no colocalization was seen in untreated cells. Taken together, these results support a scheme in which APP is concentrated in the Golgi complex as it travels through the central vacuolar system en route to the plasma membrane for secretion of its amino-terminal domain and/or to lysosomes for degradation.