1. In order to determine the presence of opioid peptides and their possible variation throughout the year in the different ganglia of the perioesophageal ring of the terrestrial snail Helix aspersa, radioimmunoassays with highly specific antisera risen for Met-enkephalin (ME) and Leu-enkephalin (LE) were carried out monthly. 2. Clear monthly variation in both IR-enkephalins' concentrations were found, following a seasonal trend. IR-LE concentrations were higher than IR-ME throughout the year in all the regions, however, no clear regional differences were found for each. 3. IR-LE/IR-ME ratios presented defined seasonal peaks for each region.
Animals Enkephalin, Leucine