The role of membrane depolarization in the regulation of expression of a neuron specific protein was evaluated by culturing superior cervical ganglia from neonatal rats in defined medium and manipulating neuronal activity by depolarizing agents. P65 is an integral membrane protein of synaptic vesicles and can be used as a marker for general neuronal maturation. P65 antigen levels were quantified by indirect radioimmunoassay, using monoclonal antibodies. The expression of p65 in ganglion explants increased by 40-100% when the cultures were treated with the depolarizing agents, veratridine or high potassium. The veratridine effect could be blocked by simultaneous treatment with the sodium channel blocker, tetrodotoxin (TTX). The rise in p65 was not evident until 36 h after depolarizing treatment had begun and reached peak levels after 48 h, with no further increases observed with sustained treatment. After removal of the depolarizing treatment, p65 levels returned to control values after 24 h. P65 joins a growing number of molecules whose expression is regulated by membrane depolarization.