Sympathetic principal neurons, dissociated from the superior cervical ganglia of newborn rats and put into culture, exhibit plasticity with respect to the choice between noradrenaline (norepinephrine) and acetylcholine as transmitter. The neurons shift from an initial, immature adrenergic state to a cholinergic state in certain culture conditions, e.g in co-culture with a variety of non-neuronal cells or after exposure to a medium conditioned by such cells. To study the transition directly, we have grown single neurons in "microcultures" with cardiac myocytes, which provide a sensitive assay for the transmitters secreted by the neurons. We have shown previously that during the transition from adrenergic to cholinergic status such neurons secrete both transmitters and have terminals of mixed fine structure (dual function). We describe here experiments in which identified neurons were serially assayed over periods of 9-45 days. Partial transitions were observed, always in the direction adrenergic to cholinergic function, and one complete transition was observed from apparently purely adrenergic function to dual function and then to apparently purely cholinergic function. We also report observation of adrenergic-cholinergic dual function, in preliminary single and serial assays, in sympathetic principal neurons from the superior cervical ganglia of adult rats.