We previously reported that microglial conditioned medium (Mic-CM) has a neurotrophic effect on cultured rat neocortical neurons [Nakajima et al. (1989): Biomed Res 10:411423]. In order to investigate the interaction between microglia and neurons in more detail, we determined the effects of Mic-CM on the primary cultured mesencephalic neurons from 16- day embryonic rats. The addition of Mic-CM to the culture medium significantly enhanced the survivability of neurons and promoted neurite extension in a low cell-density culture condition. In a high celldensity culture condition, Mic-CM markedly increased dopamine uptake, which was quantified by assessing the specific [3H]dopamine uptake, and also increased the dopamine content of cultured cells. Furthermore, the number of mesencephalic dopaminergic neurons, which was determined by quantitative analysis of tyrosine hydroxylase (TH)-immunoreactive cells, increased significantly in the presence of Mic-CM. These results suggest that Mic-CM enhances survival or maturation of TH-positive neurons present in cultures of the embryonic mesencephalon and that these neurotrophic effects may be due to a diffusible factor(s) from microglia.