Heart rate (fH), abdominal temperature (Tab) and diving depth were measured in thirteen free-ranging breeding female macaroni penguins. Measurement of these variables allowed estimation of the mass-specific rate of oxygen consumption (VO2) while diving and investigation of the physiological adjustments that might facilitate the diving behaviour observed in this species. In common with other diving birds, macaroni penguins showed significant changes in fH associated with diving, and these variables accounted for 36% of the variation in dive duration. When VO2 was calculated for dives of different durations, 95.3% of dives measured were within the calculated aerobic dive limit (cADL) for this species. Mean fH for all complete dive cycles was 147±6 beats min–1. When this fH is used to estimate VO2 of 26.2±1.4 ml min–1 kg–1 then only 92.8% of dives measured were within the cADL. Significant changes in abdominal temperature were not detected within individual dives, though the time constant of the measuring device used may not have been low enough to record these changes if they were present. Abdominal temperature did decline consistently during bouts of repeated diving of all durations and the mean decrease in Tab during a diving bout was 2.32±0.2°C. There was a linear relationship between bout duration and the magnitude of this temperature drop. There was no commensurate increase in dive duration during dive bouts as Tab declined, suggesting that macaroni penguins are diving within their physiological limits and that factors other than Tab are important in determining the duration of dives and dive bouts. Lowered Tab will in turn facilitate lower metabolic rates during diving bouts, but it was not possible in the present study to determine the importance of this energy saving and whether it is occurs actively or passively.