The small glial cells in the central nervous system of the leech, Hirudo medicinalis, have been studied using two histological stains. Weak silver carbonate, a classic stain for vertebrate microglia, can selectively stain these small glial cells and shows that they are morphologically similar to vertebrate microglia. Feulgen's DNA-specific stain is useful for counting the compact and distinctive microglial nuclei. In uninjured connectives, which link segmental ganglia, there are 134 +/- 28 microglia per 210 micron of connective length. Within 24 h after the nerve cord is crushed leech microglia aggregate at the site of injury. This increase in cells, seen both in vivo and in culture, is approximately 5-fold. Although cells do not continue to accumulate at the injury site after the first day, their numbers continue to vary with time in the regions immediately adjacent to the crush for at least one week. A second crush made 24 h after the first shows that leech microglia are capable of responding to repeated injury.