ABSTRACT: Three Steller sea lions Eumetopias jubatus, trained to dive voluntarily to depths ranging from 10 to 50 m, were used to determine whether the relationship between activity and metabolic rate during a diving interval (MR_DI, dive + surface interval) was affected by fasting (9 d) during the breeding season (spring through summer). We subsequently used the relationship between activity and MR_DI to partition the metabolic costs between underwater breath-holding activity and surface breathing activities. We estimated activity from overall dynamic body acceleration (ODBA) measured using a 3-axis accelerometer, and measured MR_DI using flow-through respirometry. The relationship between ODBA-based activity and MR_DI was not affected by fasting period, suggesting that ODBA can be used to predict energy expenditure regardless of nutritional state in the spring and summer. However, the relationship between ODBA and dive metabolic rate differs from the relationship between ODBA and the surface metabolic rate before diving. Partitioning MR_DI into the metabolic cost of remaining at the surface versus swimming underwater suggests that the metabolic cost of diving for Steller sea lions is approximately 29% lower than when breathing at the surface. ODBA appears to be a reasonable proxy to estimate metabolic rate in marine mammals, but more detailed behavioral data may be required to accurately apply the method in the field.