Seasonal alterations of serotonin (5HT) as well as the release of 5HT and the presence of the synthesizing enzyme of serotonin (5HTP-decarboxylase) were investigated in the neural sheath and desheathed ganglia of Helix pomatia. It has been established that i) serotonin concentration shows a seasonal variation in the sheath, ii) serotonin is not synthesized in the neural sheath, iii) K(+)-dependent serotonin release occurs only in the desheathed ganglia.