Intestinal blood flow interacts with intestinal absorption at several levels and affects absorption through several mechanisms. The precise relationships are difficult to define because, at present, there is not an adequate technique available for measuring blood flow to the absorptive site. The absorption of certain nutrients requires O2 delivered through the blood for maintenance of carrier-mediated transcellular transport. Both altered capillary pressure, which may or may not accompany altered blood flow because of myogenic autoregulation, and changes in absorption can influence tissue hydrostatic and colloid osmotic pressure and also epithelial and interstitial space conductance. These latter, in turn, can change the rate of passive ultrafiltration for a constant driving force. Changes in blood flow can also influence the rate of washout of absorbed substances. Countercurrent exchange in the villous vasculature can buffer the rate of absorption of some substances and may also facilitate the absorption of water. Regulatory agents such as hormones or neurotransmitters can affect blood flow and absorption both independently and interactively. Agents that cause active intestinal secretion may have their effects modified by consequent changes in blood flow.