During development, cells become progressively restricted, until they reach their final phenotype. Differentiation was originally thought to be irreversible, but phenotypic plasticity has been observed in a variety of cell types, for example sympathetic neurones, the limb blastema and some glial cell types. A detailed description of the individual steps that lead to expression or reversal of phenotype is essential to understand the molecular events underlying cell differentiation. We examined whether ciliary neurones acquire adrenergic properties when exposed to a permissive embryonic environment. Cholinergic neurones were selectively labelled with a retrogradely transported marker and injected into chick embryos during active neural crest migration. Four to five days after injection, some of the labelled neurones were found in 'adrenergic sites' and had developed catecholamine histofluorescence. The cells had thus accumulated adrenergic neurotransmitters even after differentiation into cholinergic neurones. This result shows that neurotransmitter plasticity occurs in cholinergic neurones and suggests that the neurotransmitter phenotype can be modified by the embryonic environment.