It is well documented that target cells can regulate the morphological and biochemical development of peripheral afferent neurones, but little is known about the existence of such regulatory mechanisms in the central nervous system. We therefore investigated previously the influence of striatal target cells on the maturation in vitro of nigrostriatal dopaminergic neurones, which survive in culture for more than 5 weeks, develop dense arborizations and both take up 3H-dopamine (DA) by a high-affinity specific process and synthesize 3H-DA from 3H-tyrosine. Furthermore, depolarization by potassium or veratridine stimulates the release of DA through a calcium-dependent mechanism and tetrodotoxin prevents the veratridine-evoked release of the transmitter. Both the number of 3H-DA uptake sites and the capacity for 3H-DA synthesis were at least doubled when the neurones were cultured with target cells from the striatum. To determine whether glial cells which proliferate in serum-complemented medium are partly responsible for the maturation of dopaminergic neurones and/or for the effect of striatal cells, we have now repeated the experiment using serum-free medium in which virtually pure neuronal populations can be obtained. The reduction in the number of glia did not affect either the maturation of dopaminergic cells alone, or the effect of striatal cells. Autoradiographic analysis of the number of dopaminergic cells strongly suggests that the stimulatory effect is related to increased capacities of 3H-DA uptake and synthesis per dopaminergic neurone.