A system is described for chronic cannulation of the thoracic aorta and superior vena cava in the rat. This system employs a protective stainless steel coiled spring and water tight swivel and allows chronic infusion of solutions and simultaneous intermittent blood sampling for prolonged periods of time in the undisturbed unanesthetized rat. The rats (200-250 g) lost approximately 25 g in the immediate postoperative period, but then gained weight at a relatively normal rate (cannulated rats, 2.23 g/day; control rats, 2.29 g/day).