1. A method of selecting a neurogenic agent, comprising: providing a stable human neural progenitor cell line, wherein (i) the cell line is derived from an area of the central nervous system or embryonic stem cells, (ii) the cell line has been expanded for at least ten cell doublings without substantially differentiating, and (iii) the cell line is capable of generating neurons and glia upon differentiation by mitogen withdrawal; plating cells of said cell line into a culture vessel with a serum-free, mitogen-free medium; contacting the cells with a test agent; incubating the cells in the presence of the test agent; and selecting the test agent as a neurogenic agent if said test agent increases the number or proportion of neurons by at least ten percent (10%).