In terms of a seal's success as a predator, the ability to spend a high fraction of time submerged and yet remain in a steady state is at least as important as that to make long dives. This aspect of seal physiology has received much less attention than has the ability to dive for extended periods and yet is important to the context in which dive-related morphological and physiological specializations are considered. We found, in seals exercising in the laboratory, that as [Formula: see text] increased, heart rate during breathing bouts was high and unchanging. Heart rate during dives was 25 to 33% of the rate during breathing and was insensitive to metabolic rate. However, the fraction of time seals spent breathing and overall average heart rate increased with exercise. We have confirmed these patterns using very high frequency and acoustic telemetry in freely diving harbour seals in the open sea. Heart rate at the surface was virtually constant at 120 beats/min and independent of dive duration. While the seals were diving, heart rate fell to 40 beats/min. Average heart rate depended almost entirely on the proportion of time spent below the surface and only to a minor extent on dive length. The exceedingly low heart rates seen during forced dives in the laboratory were never observed at sea.