The ability and persistence of salmonids in ascending pool-and-overfall fishways was measured in experimental “endless” fishway structures in which fishways of any height could be simulated. Six chinook salmon (Oncorhynchus tshawytscha), four sockeye salmon (O. nerka), and four steelhead trout (Salmo gairdneri) were permitted to ascend 1,000 feet in the experimental fishways. One sockeye salmon was allowed to ascend an endless fishway for over 5 days, climbing 6,648 feet, a vertical ascent of more than a mile, before the test was discontinued. Rate of ascent of all fish tested increased following an initial period of experience in the fishway. The level of blood lactate in the exercised fish showed no evidence of fatigue. Practical significance of the data in relation to fishway design is discussed.