In the present investigation the action of 6-hydroxydopamine (6-OHDA) on Auerbach's plexus of the ileum of the guinea pig has been studied with Falck and Hillarp's histochemical technique. A singel dose of 15-25 mg/kg of 6-OHDA causes degeneration of some of the adrenergic nerve fibres after 6 h of treatment. 200-250 mg/kg of 6-OHDA is responsible for the total disappearance of nerve fibres in the ganglia and internodal strands within 24 h. 6 h after 100 mg/kg of 6-OHDA treatment, the intervaricose fibres have mostly disappeared and a few nerve fibres are swollen. The pre-terminal fibres are not much affected. After 24 and 48 h of 100 mg/kg treatment with 6-OHDA, most of the varicose and non-varicose fibres have disappeared and very few fibres are left, these having become intensely fluorescent and much swollen, probably due to the accumulation of storage granules transported to this site by axoplasmic flow. After 6-8 days of treatment with 6-OHDA all the fluorescen fibres and cell bodies have disappeared. A few faintly fluorescent green and yellow cell bodies could also be observed; however, their chemical nature could not be determined.