Incubation of synaptosomes under conditions which result in complete phosphorylation of membrane bound accepter proteins does not affect the permeability to Na + or K + as measured by a spectrophotometric method. This technique was not, however, sensitive enough to determine permeability to Ca 2+ which was thus estimated using 45Ca 2+. It was found that although phosphorylation did not affect the equilibrium binding of 45Ca it did lower the rate of both Ca 2+ uptake and efflux. The most likely interpretation of these results is that phosphorylation of proteins in the synaptic membrane lowers the permeability of the membrane to Ca 2+. This could have a role in the regulation of synaptic transmission.