The myenteric plexus of the guinea pig ileum was studied in stretch preparations of the longitudinal muscle layer with adherent plexuses, and in freeze-dried transverse sections from the small intestinal wall. Catecholamines and serotonin (5-HT) were visualized according to the Falck-Hillarp technique. Emission spectra from the resulting fluorophores and recordings of their rates of photodecomposition were analysed. Adrenergic nerve terminals showed a slow fluorescence fading rate and a fluorescence spectrum compatible with their known contents of noradrenaline (NA), while the enterochromaffin cells showed a rapid exponential fading and a fluorescence spectrum compatible with their known contents of 5-HT. In order to unmask any low amounts of 5-HT in the neurons of the plexus, analysis of fluorescence parameters at various time intervals after pretreatment with reserpine followed by MAO-inhibition was performed. With the methods used no evidence of the presence of 5-HT in the myenteric plexus of the guinea pig could be found.