Subcellular fractionation studies of rat brain homogenate revealed that peptide histidine isoleucine-like immunoreactivity (PHI-LI) was enriched in synaptosomal preparations. PHI-LI was released by high potassium concentrations from synaptosomal pellets incubated in vitro. These results suggest a possible physiological role of PHI in the central nervous system as a neurotransmitter or modulator of synaptic function.