Embryonic neural precursors (ENPs), also termed neural stem cells or "neurospheres," are an attractive potential source of tissue for neural transplantation, because of their capacity to expand in number in vitro while retaining the ability to develop into the major phenotypes of the CNS. ENPs are isolated from the developing brain and proliferate in the presence of mitogens such as FGF-2 and EGF. Subsequent withdrawal of these mitogens and exposure to a suitable substrate results in differentiation into the major cell types of the CNS. As well as its role in precursor cell expansion, FGF-2 also plays a key role in the division of astrocytes, and in neuronal differentiation. Thus, it is important to establish the optimal concentrations of this factor for expansion and differentiation of neuronal phenotypes. Here we explore the effect of FGF-2 concentrations ranging from 1 to 20 ng/ml on the expansion and differentiation capacity of ENPs isolated from the cortex and striatum of E14 mice. ENP expansion was seen under all conditions, but was greatest at 10 and 20 ng/ml and least at 1 ng/ml. The numbers of neurons (as a proportion of total cell number) differentiating from these ENP populations appeared to be greatest at 1 ng/ml. However, once adjustments were made for the amount of expansion at each dose, final neuronal yield was maximum at the highest concentration of FGF-2 used (20 ng/ml).