Manipulation of the cellular and hormonal environment of cultures of dissociated primary neurones can be used to explore a neurone's developmental potential and to investigate the factors required for normal development. For example, developing adrenergic sympathetic neurones can be influenced to become cholinergic by both diffusible¹ and membrane-bound² factors from certain types of non-neuronal cells; when medium conditioned by incubation on heart cell cultures (CM) is placed on the neurones, they develop the ability to produce acetylcholine (ACh) and they form functional cholinergic synapses with each other³. Hormones could also contribute to the control of this transmitter choice⁴, and McLennan et al.⁵ recently reported that corticosterone treatment of whole superior cervical ganglia (SCG) greatly inhibited the cholinergic development of these ganglia in culture. It was not clear, however, whether the hormone acted directly on the neurones or indirectly via the non-neuronal cells. To study the role of hormones on this transmitter choice, I have now developed a serum-free medium (based on that of Sato and co-workers^6,7) for the preparation of conditioned medium. The results obtained with this system show that glucocorticoids and epidermal growth factor (EGF) exert dramatic and antagonistic effects on the adrenergic–cholinergic transmitter choice and do so indirectly, by controlling the ability of heart cells to produce cholinergic CM.