Poor stirring of intestinal contents yields a preepithelial diffusion barrier that is thought to be the rate-limiting step in absorption of many compounds. In many previous studies, the resistance of this barrier is equal to an unstirred water layer of 300-800 micron. Using three probes, CO, glucose, and [14C]warfarin, we measured the preepithelial resistance in a 30-cm segment in rats that were 1) conscious, 2) anesthetized with pentobarbital sodium, or 3) anesthetized and laparotomized. Measurements with each of the probes showed that the maximal preepithelial resistance in conscious rats was equivalent to an unstirred layer of only approximately 100 micron. Anesthesia roughly doubled this resistance, and anesthesia and laparotomy caused a sixfold increase (unstirred layer of approximately 600 micron). We conclude that the luminal stirring is much more efficient than previously has been appreciated. The very thick jejunal unstirred layers reported previously (300-800 micron) reflect the results of studies performed under nonphysiological conditions or studies employing inappropriate techniques to measure luminal stirring.