The electron microscopic studies of the rat brain neocortex and neurosecretory structures have revealed that after the intensification of calcium metabolism by parathormone injection the destructive changes mainly occur in the structures involved in the maintenance of intracellular calcium homeostasis (mitochondria and microvesicles). The morphofunctional signs of decreased axonal terminal activity have been also shown by their over-excitability and suppression of conductivity.