Cardiac output was determined by the thermodilution technique in three California sea lions while resting and while swimming. Metabolic rates increased sevento ninefold above resting rates during maximal exercise. While the sea lions were at rest, stroke volume was also determined by simultaneously counting heart rate during cardiac output determinations. At rest, cardiac output (2.5-3.0 mL kg⁻¹s⁻¹) and stroke volume (2 mL kg⁻¹) were similar to those of harbor seals and terrestrial mammals of similar mass. During exercise, mean cardiac output increased linearly with work load and surface/submerged intervals were short and frequent. The exercise capacity of swimming sea lions appears similar to that of harbor seals, but the exercise response resembles that of terrestrial mammals more than that of harbor seals.