A new method of mounting a microdrive on the skull and adjusting the trajectories of microelectrodes is described. The key to this system is a swiveling guide tube held in a small, skull-mounted base by a low-melting-point metal alloy. The microelectrode is advanced via a modified, commercially available, miniature microdrive screwed onto the guide tube. To change the trajectory of the electrode, the alloy is melted in place, and the guide tube swiveled to a different angle. The microelectrode or the trajectory of the pass may be changed in a few minutes while easily maintaining aseptic conditions. The entire assembly is small enough so that several can be simultaneously implanted on the skull of a cat. A metal crown is used to fix the head during recording sessions or to hold a fiberglass cap that protects all skull implants between sessions. Since the microdrives need not be removed between recording sessions, an electrode pass may be continued from day to day. Although guide tubes are generally employed only for subcortical structures, this arrangement also works well when recording from cortex in deeper parts of gyri.