Whether blood mononuclear cells are totipotential and can give rise to fibrocytes is still controversial. Most recent studies seem to favor that fibrocytes arise locally from perivascular resting undifferentiated cells. We have attempted to determine the origin of fibrocytes utilizing tritiated thymidine (3HT) autoradiography. Multiple skin incisional wounds were produced in the dorsa of rats and a subsequent single intraperitoneal injection of 3HT was given at varying periods after injury. Biopsies were taken from individual animals at varying intervals up to 192 h after injury. Sections representing biopsies up to 15.5 h after injury, taken from animals injected at 4, 7, 8, and 11.5 h after injury, did not show labeling of monocytes, fibrocytes or endothelial cells. Biopsies taken from these same animals at varying intervals after 16 h showed labeling of these cells. These observations indicate that new-labeled fibrocytes which appeared in the wound were labeled at a distant site since labeling did not occur locally 1 h after a pulse injection of 3HT. Our findings thus support the concept that blood-borne mononuclear cells can give rise to some of the fibrocytic cells in repair of wounds.