The distributions of tubulin and high molecular weight microtubule-associated proteins (HMWPs) in brain were determined by immunoperoxidase histochemistry with specific antisera. Tubulin was found in microtubules of both neurons and glia and both axons and dendrites. HMWPs were found only in neurons where, in all cases examined, they were associated with dendritic microtubules but not those in axons. Both tubulin and * H HMWPs were also found in postsynaptic densities. These results indicate that brain microtubules vary in chemical composition. The preferential association of HMWPs with dendritic microtubules suggests that they may play a role in distinguishing between dendritic and axonal export routes from the cell body.