Some modifications to the method of PONZ et al. for in vivo intestinal absorption studies using an in situ perfused segment of small intestine, under anesthesia, are described. They improve its accuracy and applicability, especially when slowly absorbed substrates are used or when volume flux measurements are desired. Calculations for the absorbed substrate and net fluid volume change determinations are reported. Several aspects of the use of the method under a single pass or a recirculation perfusion system are discussed.