A relationship between L-phenylalanine and L-serine absorption, intestinal motility and blood flow has been studied with a canine in situ isolated jejunal loop in acute experiments and in chronic preparations in conscious dogs. During spontaneous rhythmic contractions, as well as after mechanical and pharmacological stimuli, rhythmic changes in blood flow, related to intestinal contractions, were observed. They had no bearing on mean blood flow or absorption as long as they did not occur together with tonic contractions. The tonic contractions resulted in a prolonged decrease in jejunal blood flow ano diminished amino acid absorption rate. Absorption and motility index, as well as blood flow and motility were negatively correlated. There was a positive rectlinear correlation between intestinal blood flow and absorption, regardless of whether changes in blood flow resulted from the increase in motility or were induced by intestinal artery occlusion. These correlations suggest that changes in amino acid absorption during increased motility depend on changes in blood flow. This relationship may be important in clinical syndromes with hypermotility.