Heart rate (fH) and rate of oxygen uptake (VO2) were measured in six subadult California sea lions Zalophus californianus while they were at rest and while they were swimming for 15 min at controlled speeds of up to 1.4 m s-1 and pulling loads of up to 3 kg. There was a good linear relationship between fH and VO2 in all six sea lions. The slopes of the individual regression lines varied between 2.66 and 4.36 beats ml-1 O2 kg-1, the intercepts varied between 48.2 and 63.0 beats min-1 and r2 varied between 0.82 and 0.93. The mean relationship for all six sea lions is fH = (57.4 +/− 2.0) + (3.58 +/− 0.23) VO2, r2 = 0.89 +/− 0.01. The mean of the lowest VO2 values was 5.1 +/− 0.4 ml min-1 kg-1 and the mean of the highest VO2 values was 26.9 +/− 1.9 ml min-1 kg-1. The means of the lowest and highest values of fH were less extreme, being 72 +/− 3 beats min-1 and 155 +/− 5 beats min-1, respectively. It is concluded that, by using data storage devices and grouped data, fH could be used in otariids as an indicator of aerobic metabolism under field conditions, in particular for breeding females during the period of lactation.

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