The midgut of the corn earworm, Helicoverpa zea, contains endocrine cells which exhibit immunoreactivity resembling Phe-Met-Arg-Phe-NH2 (FMRFa), a molluscan cardioactive peptide and a member of a recognized family of molecules termed RF-amide peptides. An extract of 10,000 midguts was fractionated by HPLC and FMRFa immunoreactivity was determined by radioimmunoassay (RIA). Two peptides were isolated and their sequences determined by tandem mass spectroscopy were: Gln-Ala-Ala-Arg-Pro-Arg-Phe-NH2 and Ala-Ala-Arg-Pro-Arg-Phe-NH2. These new peptides are termed Hez-MP-I and Hez-MP-II, respectively. Their sequences resemble the carboxyl terminal tetrapeptide of (a) the pancreatic polypeptides of lower vertebrates and (b) a related neuropeptide from squid. An antiserum was raised against Hez-MP-I to develop a specific RIA, which was used with HPLC to demonstrate that each of these peptides occur in hemolymph, as well as in midgut and brain. Hez-MPs thus qualify as putative hormones which may play a role in coordination of digestion in this insect.