To investigate the amino acid requirements of the senescent rat, as part of a study directed toward nutritional support in the aged, it was necessary to determine amino acid levels in plasma and tissue, but also regional blood flow of the animals subjected to fast. Only this latter allows the determination of the amounts of each amino acid present in the tissue before starvation by extrapolation of values measured during starvation. As plasma and tissue amino acid had been previously determined, the aim of this study had been to measure regional blood flow in the liver, kidney, testis, spleen, stomach, small intestine and large intestine in senescent rats submitted to 1, 5, 9 and 15 days of starvation. Twenty-four-month-old male Wistar rats (n = 16) were divided into four groups (n = 4), and submitted to starvation for 1, 5, 9 and 15 days. Blood flow in the liver, kidney, testis, spleen, stomach, and small and large intestine was measured by injecting 0.5 ml of a microsphere solution (15 microns diameter) labelled with 57Co, 0.25 microCi/ml. Over the 15-day period studied, the response to starvation showed two distinct phases: an early effect (from day 1 to day 9) in which there were decreases in the weight of the organs and in organ blood flow, and a second phase (from day 9 to day 15) in which blood flow and organ weight were maintained. However, organ blood flow related to mass was not substantially affected by starvation. This implies that measurement of substrate plasma concentration alone can reliably reflect organ substrate flow.