Many sympathetic principal neurones of the superior cervical ganglion of the newborn rat are known to be plastic with respect to the choice between norepinephrine (NE) and acetylcholine (ACh) as transmitter; when the neurones are dissociated and placed in culture, a majority of them can be shifted from an initial, immature, adrenergic state to a cholinergic state by co-culture with a variety of non-neuronal cells or by medium conditioned by such cells. To study this transition it has been helpful to grow single neurones, each in a microculture which also contains cardiac myocytes. The transmitter status of a neurone can be assayed by recording its effect on the myocytes (adrenergic excitation, cholinergic inhibition or dual function); then a fine structural assay of the neurone based on the appearance of the synaptic vesicles can be made and correlated directly with the physiology. In this paper we report the following findings on principal neurones developing in such microcultures. (i) During the transition period, a majority of the neurones were dual in function and in vesicular appearance. (ii) The physiological effects and vesicular appearance varied from mainly adrenergic to mainly cholinergic. (iii) In preliminary attempts to follow the transition by recording at least twice from the same microculture, partial transitions were observed, always in the direction adrenergic-to-cholinergic. (iv) The transitions were not synchronous or fixed in time course even in pairs of neurones grown side by side in the same microculture.