Pressure-flow relationships (P/F) in the small intestine or hindlimb circulation were used to examine autoregulation in 104 swine, aged 1 day to 2 months and anesthetized with pentobarbital. The P/F were obtained while perfusion pressure was decreased by compression of the abdominal aorta for 2 min at each pressure. Readjustments in vascular resistance stabilized within 2 min. Intestinal flow decreased with each decrease in pressure in swine 2 weeks old or younger, but was well maintained by 1 month of age. Femoral flow decreased with each decrease in pressure at all ages studied. Control experiments indicated that the observed vascular responses were independent of angiotension-II activity and were not reflex in origin.