Abstract: Microdrive and modular microdrive assembly for positioning the tips of substantially rigid medical and scientific instruments, such as electrodes, mechanical probes and needles, that are chronically implanted in animals, especially conscious and freely-moving animals, without passing the substantially rigid instruments through tubular guides or using immobilizing stereotactic surgical guide systems, such as arcuate rails mounted on relatively large and unwieldy external surgical headframes. Embodiments of the invention comprise a bottom plate adapted to be surgically attached to the animal and fixedly secured to a frame so that a void in the bottom plate is fixedly disposed over the implantation site. A carriage having a platform for mounting the substantially rigid instrument is alternately lowered and raised by rotating a drive rod connecting the carriage to the frame, enabling precise movement of the tip of the substantially rigid instrument into and out of the animal's body tissue.