We have studied the effects of basic fibroblast growth factor on rat embryonic mesencephalic neurons in vitro. Basic fibroblast growth factor promotes the survival of dopaminergic neurons in vitro, the effect increasing with dose and reaching a maximum at 10 ng/ml. In the absence of basic fibroblast growth factor the number of tyrosine hydroxylase-stained (tyrosine hydroxylase positive) neurons declines to almost zero within 14 days, whereas in the presence of basic fibroblast growth factor numbers remain almost constant from three to 28 days in vitro. This effect of basic fibroblast growth factor is abolished by preventing non-neuronal cells from appearing in the cultures, apart from a basic fibroblast growth factor-mediated increase in the numbers of tyrosine hydroxylase-positive cells during the first two days in vitro. The presence or absence of non-neuronal cells also influences dopaminergic neuronal morphology, the neurons having more, longer, and more varicose processes in the absence of astrocytes. Survival of dopaminergic neurons in vitro in the absence of basic fibroblast growth factor is very dependent on plating cell density, but in the presence of basic fibroblast growth factor this dependency vanishes. It is also possible to make survival independent of plating density by growing the cultures on inverted coverslips, which have the effect of concentrating secreted molecules in the thin layer of medium between coverslip and dish. Our conclusions from these experiments on plating density are that astrocytes probably constitutively secrete a small amount of a trophic factor which promotes survival of dopaminergic neurons, and that the rate of production of this factor is greatly increased by basic fibroblast growth factor. If basic fibroblast growth factor is withdrawn from cultures after two or seven days the dopaminergic neurons soon die. However, if basic fibroblast growth factor is withdrawn after 14 days, after the period of naturally occurring cell death of these neurons, there is no increase in dopaminergic neuronal death compared to controls in which basic fibroblast growth factor treatment is maintained. If basic fibroblast growth factor is used to improve the survival of dopaminergic neurons grafted in vivo, it should therefore be sufficient to treat the grafts for 14 days.