The present study is the first to simultaneously and continuously measure oxygen consumption (MO(2)) and gastrointestinal blood flow (q(gi)) in fish. In addition, while it is the first to compare the effects of three isoenergetic diets on q(gi) in fish, no significant differences among diets were found for postprandial MO(2), q(gi) or heart rate (f(H)) in rainbow trout, Oncorhynchus mykiss. Postprandial q(gi), f(H) and MO(2) were significantly elevated above baseline levels by 4 h. Postprandial q(gi) peaked at 136% above baseline after 11 h, f(H) peaked at 110% above baseline after 14 h and MO(2) peaked at 96% above baseline after 27 h. Moreover, postprandial MO(2) remained significantly elevated above baseline longer than q(gi) (for 41 h and 30 h, respectively), perhaps because most of the increase in MO(2) associated with feeding is due to protein handling, a process that continues following the absorption of nutrients which is thought to be the primary reason for the elevation of q(gi). In addition to the positive relationships found between postprandial MO(2) and q(gi) and between postprandial MO(2) and f(H), we discovered a novel relationship between postprandial q(gi) and f(H).