Dorsal aortic blood pressure, heart rate, and oral water pressure were recorded in adult sockeye salmon (Oncorhynchus nerka) while they were resting in darkened aquaria or swimming in an illuminated tunnel-respirometer. Pressures were recorded on an oscillograph attached to pressure transducers connected with cannulae in the dorsal aorta and oral cavity. The velocity of the water in the swimming chamber was increased in stepwise fashion every 30 min until the fish stopped swimming and were swept against an electrified grid. At the cessation of swimming, which was considered as the onset of fatigue, water velocity was decreased to a minimum which permitted the fish to rest and dorsal aortic blood pressure, heart rate, and oral water pressure were recorded for about 1 hr as they returned toward resting levels. Resting values were 44/38 mm Hg (systolic/diastolic) and 49 heartbeats/min. At maximal swimming speed dorsal aortic blood pressure was 55/49 mm Hg and heart rate was 83 beats/min. No abrupt changes in cardiovascular function occurred at the onset of fatigue.A number of morphological (length and weights of the body and ventricle) and cardiovascular (pressures, rates) variables were measured in six adult sockeye salmon. A significant correlation was found between hematocrit, ventricle weight, and heart rate; a change in one apparently caused adjustments in the other two in order to maintain a constant supply of oxygen to the tissues during subfatigue levels of activity.The effects of removal and replacement of blood or isotanic saline in the dorsal aorta were observed in three adult sockeye salmon. Removal of blood produced a proportionate decrease in blood pressure and an increase in heart rate. Neither dilution of the blood by replacement with saline nor increase in blood volume from addition of saline had any significant effect on heart rate and blood pressure.