Protein phosphorylation represents an approach, sometimes the only approach available, to study the molecular basis for a wide variety of neurophysiological phenomena. The injection of protein kinases or protein kinase inhibitors into neurones has provided direct evidence that activation of protein kinases has an obligatory role in the mechanisms by which numerous extracellular signals produce specific physiological responses in neurones. A diversity of substrate proteins for the kinases have already been found. In several instances, the identity and functional role of these substrate proteins have been established.