Phosphoproteins and their associated kinases and phosphatases play a major role in second messenger signal cascades in neurons. Immunocytochemical studies have shown that some of these phosphoproteins and their associated enzymes are homogeneously distributed in brain and function in signal cascades common to all neurons. Other phosphoproteins and their associated enzymes are heterogeneously distributed and function in signal cascades related to specific neuronal subpopulations. Phosphoproteins and their kinases and phosphatases have been localized in the brain by electron microscopic immunocytochemistry, allowing their subcellular distributions to be analyzed. A sensitive technique employing avidin, biotin, and horseradish peroxidase has been especially useful in this endeavor. This method, along with suggestions for optimizing fixation and antibody incubations, is summarized.