Enkephalin, applied microiontophoretically, depressed spontaneous and glutamate-induced firing of single neurons in frontal cortex, caudate nucleus, and periaqueductal gray matter, where enkephalin and high concentrations of opiate receptors are found. Many of the depressions were blocked by the specific narcotic antagonist naloxone. The data are compatible with a neurotransmitter or neuromodulator role for this new brain pentapeptide.