Repetitive stimulation (100 pulses per second for 1 second) of the Schafer collateral-commissural system of the rat hippocampus induces long-term potentiation of synaptic strength and produces significant changes in the subsequent endogenous phosphorylation of a 40,000-dalton protein from synaptic plasma membranes. This effect is not observed after stimulation in calcium-deficient media or after simulation at the rate of one pulse per second for 100 seconds. These findings provide evidence that repetitive synaptic activation can alter the phosphorylation machinery of the synaptic region and suggest a biochemical process which may be involved in the production of neuronal plasticity.