The microvessels responsible for the major decrease in intestinal vascular resistance during the presence of glucose were defined. In addition, the normal distribution of tissue PO2 in the various layers of the intestinal tissue was measured at rest and during glucose exposure to determine if part of the absorptive hyperemia mechanism is related to a decrease in tissue PO2. In the rat small intestine, exposure of the mucosa only to glucose concentrations of 25--500 mg/100 causes a 20--25% dilation of all submucosal vessels in series with the mucosal vessels and mucosal arterioles. Total intestinal blood flow increased to 200-210% of control at all glucose concentrations. The tissue and perivascular PO2 in the villus apex decreased from 14.8 +/- 1.2 (SE) mmHg at rest to 6--8 mmHg during glucose exposure; the PO2 in the muscle and submucosal layers tended to slightly increase above a normal of 26.4 +/- 1.6 mmHg during glucose exposure. The data indicate virtually all intestinal arterioles are equally involved in absorptive hyperemia. The dilation of mucosal vessels may be related to a decrease in tissue PO2, but submucosal vessels dilate even though PO2 is slightly increased.