Several lines of evidence indicate that enkephalin-like peptides in adrenal gland can be released into circulation. In order to explore the role of these released peptides, the stability of the enkephalin-like peptides extracted from adrenal glands in plasma was studied. The high molecular weight enkephalin-like peptides were found to be very stable, while the low molecular weight enkephalin-like peptides were rapidly degraded in plasma. Our preliminary result also suggests that there may be an enkephalin binding peptide or protein in plasma. Hence it is possible that enkephalin may be released into circulation in some kind of protected form such as bound to a large peptide or protein. In fact presence of enkephalin in blood has been documented in spite of its very short half-life. We are planning to explore the possible target tissues for both high and low molecular weight forms of enkephalin-like peptides in periphery.