A method is described that allows high-frequency venous and arterial blood sampling in the laboratory rat. After distal ligation and division of the external jugular vein and carotid artery, sections of Teflon intravenous catheters are inserted and secured with circumferential ligatures. Between these Teflon segments, a vascular circuit is maintained with the interposition of silicone tubing. The silicone tube segment has been filled with heparinized saline, divided in half, and joined at its center by another segment of the catheter. This in situ arteriovenous (AV) shunt system is secured in the neck of the host animal by creating a subcutaneous tunnel and externalization of a portion of the shunt loop. The shunt facilitates repetitive blood sampling from the conscious rat. The procedure time is approximately 45 minutes, and patency can be assured over several days. Additional uses of the system include pretreatment injection protocols and postoperative drug therapy.