The regrowth of central dopamine-, noradrenaline-, acetylcholine-, and GABA-producing neurones after axonal damage has been studied using transmitter related enzymes as markers for regenerating sprouting fibres. Irides, implanted into the stratum, the anterior hippocampus, and the anterior eye chamber, were used to ‘trap’ the nerve fibres sprouting in the lesioned area. After a survival time of 3–84 days the implants were removed and assayed for the specific activities of tyrosine hydroxylase (TOH) and aromatic amino acid decarboxylase (AAD) as markers for dopaminergic and noradrenergic fibres, choline acetyltransferase (ChAT) and acetylcholine-sterase (AChE) as markers for cholinergic fibres, and glutamic acid decarboxylase (GAD) as marker for GABA-producing fibres. As it was removed from the eye, the iris transplant was completely denervated of its normal adrenergic and cholinergic nerve supply. As a consequence the levels of TOH, AAD and ChAT dropped to about 5–15% of normal within 3–5 days after transplantation. As the new sprouts grew into the iris from the brain or anterior eye chamber the enzyme levels gradually recovered. The most rapid and extensive recoveries of the transmitter synthetic enzymes were observed inthe striatal implantation site. By 2–3 weeks after transplantation the ChAT activity had reached near normal level and the TOH and AAD levels were 280 and 150%, respectively, above normal. The parallel histochemical analysis revealed a regrowth of new sprouts from the so-called striatal radiation of AChE-positive fibres anf from the dopamine-containing nigrostriatal fibres. In the hippocampal implantation site the ChAT level recovered to about 50% of normal at 3 and 6 weeks, TOH to 80% at 3 weeks and 150% at 6 weeks, and AAD to 50% at 2 weeks and 65% at 6 weeks. This was paralcentral and peripheral neurones in their responses to axonal injury. Our present data support this view and provide further evidence for the high regenerative capacity of central aminergic, cholinergic, and possibly also GABA-containing neurones. Moreover, the capacity for regenerative sprouting does not seem to be a special property of certain particular CNS regions. Regeneration is most probably shared by many central non-myelinated systems and could, therefore, be a wide-spread mechanism underlying functional recovery in the aduly mammalian CNS.