The distribution and regulation of the state of phosphorylation of Protein I have been studied in the rabbit superior cervical sympathetic ganglion. The data indicate that the ganglion contains two pools of Protein I: a presynaptic pool that represents 60% of the total ganglion Protein I and a postsynaptic pool that represents 40% of the total ganglion Protein I. The state of phosphorylation of presynaptic Protein I, but not that of postsynaptic Protein I, is regulated by nerve impulse conduction, by dopamine, and by a high K+ concentration. Studies of the extracellular calcium requirements for Protein I phosphorylation, as well as peptide-mapping analyses of Protein I, suggest that the effects of nerve impulse conduction and of a high K+ concentration are mediated through the activation of calcium-dependent protein kinases and that the effect of dopamine is mediated through the activation of cyclic AMP-dependent protein kinase. The total amount of postsynaptic Protein I, but not that of presynaptic Protein I, is decreased by short periods of exposure to cycloheximide, a protein synthesis inhibitor. It is proposed that Protein I located in presynaptic nerve terminals plays a functional role in those terminals and that the Protein I located in cell bodies is newly synthesized and en route to nerve terminals.