The hypothesis that juvenile anadromous salmonids which are migrating to sea have higher levels of gill (sodium and potassium) adenosine triphosphatase ([Na+K]-ATPase) activity than nonmigrants was examined for wild and hatchery spring and fall chinook salmon (Oncorhynchus tshawytscha) from the Rogue River, Oregon. Although migrant juvenile chinook salmon captured at river km 173 had elevated gill (Na+K)-ATPase activity, migration from the more upstream portions of the Rogue River occurred without significant changes in gill (Na+K)-ATPase activity. High water flows stimulated migration without increasing gill (Na+K)-ATPase activity. Tributary-to-main-stem migration of juvenile fall chinook salmon did not result in elevated gill (Na+K)-ATPase activities. Juvenile spring chinook salmon released from Cole Rivers Hatchery (km 254) migrated rapidly to km 18 on the Rogue River without changes in gill (Na+K)-ATPase activity. We conclude that, although the relationship between seaward migration and elevated gill (Na+K)-ATPase activity is demonstrable in chinook salmon in many cases, such elevated ATPase activity is not a prerequisite for seaward movement of juvenile chinook salmon.