An apparatus and technique for the perfusion of a specific cannulated segment of small intestine of the rat, in situ, have been described. By the removal and analysis of perfusate samples from the system, continuous intestinal absorption curves have been established from a single animal in the steady state. These data agree with load determinations. Data based upon the intestinal uptake of glucose and xylose show that this system lends itself to in situ studies on absorption. Submitted on November 15, 1956