The effect of 10-10 M parathyroid hormone (PTH) on Ca2+ levels and cAMP and cGMP contents in peripharyngeal ganglions of the snail Helix pomatia was studied by radioisotope and radioimmune methods; also, chemoreception of GABA was determined in the neuronal membrane. The levels of Ca2+ and cyclic nucleotides were increased in the ganglions. Co-addition of PTH with compounds increasing the levels of Ca2+ and cyclic nucleotides lowered these elevations of the levels of Ca2+ and cyclic nucleotides. Depending on the initial levels of Ca2+ and cyclic nucleotides in the neuron, PTH can direct the fluxes of ions either into or out of the cell and can activate the cyclase or phosphodiesterase systems. PTH decreased the binding of GABA by the ganglions at high GABA concentrations and under conditions which promote increased binding of GABA. These data suggest that PTH has neuroprotective effects and protects the neuronal tissue from inhibition. Thus, PTH can modulate the function of neurons.