The New Zealand sea lion, Phocarctos hookeri, is the deepest- and longest-diving sea lion. We were interested in whether the diving ability of this animal was related to changes in its at-sea and diving metabolic rates. We measured the metabolic rate, water turnover and diving behavior of 12 lactating New Zealand sea lions at Sandy Bay, Enderby Island, Auckland Islands Group, New Zealand (50 degrees 30'S, 166 degrees 17'E), during January and February 1997 when their pups were between 1 and 2 months old. Metabolic rate (rate of CO(2) production) and water turnover were measured using the (18)O doubly-labeled water technique, and diving behavior was measured with time/depth recorders (TDRs). Mean total body water was 66.0+/-1.1 % (mean +/- s.d.) and mean rate of CO(2) production was 0. 835+/-0.114 ml g(-)(1 )h(-)(1), which provides an estimated mass-specific field metabolic rate (FMR) of 5.47+/-0.75 W kg(-)(1). After correction for time on shore, the at-sea FMR was estimated to be 6.65+/-1.09 W kg(-)(1), a value 5.8 times the predicted standard metabolic rate of a terrestrial animal of equal size. The mean maximum dive depth was 353+/-164 m, with a mean diving depth of 124+/-36 m. The mean maximum dive duration was 8.3+/-1.7 min, with an average duration of 3.4+/-0.6 min. The deepest, 550 m, and longest, 11.5 min, dives were made by the largest animal (155 kg). Our results indicate that the deep and long-duration diving ability of New Zealand sea lions is not due to a decreased diving metabolic rate. Individual sea lions that performed deeper dives had lower FMRs, which may result from the use of energetically efficient burst-and-glide locomotion. There are differences in the foraging patterns of deep and shallow divers that may reflect differences in surface swimming, time spent on the surface and/or diet. Our data indicate that, although New Zealand sea lions have increased their O(2) storage capacity, they do not, or cannot, significantly reduce their at-sea metabolic rates and are therefore likely to be operating near their physiological maximum.