1. The release of the crustacean hyperglycemic hormone (CHH) is inhibited by synthetic Leu-Enkephalin (L-Enk). 2. Endogenous enkephalinergic material was isolated by reversed phase liquid chromatography. 3. The endogenous substance inhibits CHH release as demonstrated by bioassay in vivo and in vitro. 4. The decrease in CHH release is antagonized by naloxone. 5. L-Enk immunoreactivity is demonstrated in the eyestalk of Carcinus maenas. 6. The results suggest a modulatory role of L-Enk in the control of neuropeptide release.