Neurolucida is a new software package for performing 3-D neuron mapping and tracing to 0.5 micron precision through the oculars of a microscope. It incorporates computer controlled stage movement in three dimensions. Sections are limited in size only by the stage dimensions. An on-line, direct view image editor is provided as are dynamic image rotations in color and morphometric analyses. Neurolucida is written in 'C' and runs on PC ATs, XTs, and PS/2s using DOS and VGA, EGA, or NTSC graphics. Neurolucida has a video mode compatible with frame grabbers, thereby permitting its application to video microscopy.