STUDIES of the cellular reaction of peripheral and central catecholaminergic neurones to axonal injury (the retrograde reaction and axon response) have demonstrated that such lesions elicit a prolonged reduction within the parent cell body in the activities and amounts of the catecholamine biosynthetic enzymes tyrosine hydroxylase (TH) and, in noradrenergic neurones, dopamine-β-hydroxylase (DBH)^1–3. The reduction in enzyme protein, usually reversible, lasts for several weeks and is apparently due to a decrease in the rate of enzyme biosynthesis⁴. It has therefore been proposed that a reduction in amount and activity of neurotransmitter biosynthetic enzymes may be a biochemical indicator of the retrograde reaction³. However, it remains to be established if a reversible diminution of neurotransmitter biosynthetic enzyme occurs during the retrograde reaction in neurones which synthesise neurotransmitters other than catecholamines. This is suggested from studies of the retrograde reaction in hypoglossal neurones, cholinergic motor neurones which innervate the tongue through the hypoglossal nerve. During the retrograde reaction elicited by lesions of the hypoglossal nerve, the activities of choline acetyhransferase (CAT), the enzyme synthesising acetylcholine, and acetyl-cholinesterase (AChE), the enzyme degrading it, are markedly reduced^5–7. If the proximal stump of the lesioned nerve is allowed to regenerate and reestablish synaptic contact, AChE activity within the hypoglossal nucleus rapidly reappears⁷. It is, however, unknown whether the reduction in CAT activity is reversible and, because of a lack of specific, inhibiting antibody to CAT, whether the reduction is due to a decrease of enzyme molecules. We report here that the reduction in CAT in hypoglossal neurones elicited by transection of the hypoglossal nerve is reversible and also, by use of a specific antibody, that it is due to a decrease in the amount of CAT enzyme protein.