The objective of this study was to assemble the components of an energy budget for the harbour porpoise in the western Bay of Fundy, Canada. The annual energetic requirements for basal and active metabolism were estimated to be about 902250 kcal, food intake (at 1.78 kg/24 h), was based on stomach-content data for days with one flood-tide period in daylight and was assumed to be twice that on the 74 days of summer when two such periods occur. Radio-telemetry indicated relatively little activity at night. The annual intake was calculated to total about 982000 kcal after correction for a net assimilation rate of 0.74. Measurements of heat flux through skin and blubber predicted total heat loss of about 814000 kcal/annum. All three of the annual summations were calculated by independent methods, yet all fell within 8% of the mean value. We concluded, contrary to some statements in the literature, that the harbour porpoise conforms to the BMR predicted by the equation of Brody (1945) and is sufficiently insulated for the average temperature of 8°C (range 2-15 °C) which it encounters in the region. The daily feeding rate of a non-lactating adult would seem to be only about 3.5 % of total body weight per day. The rate of 8.67 %/day in the literature was based on captive specimens and is believed to result from a combination of obligate opportunistic feeding and unnatural conditions in captivity. The total fat store is about 150000 kcal on average; the cost of pregnancy can be met from the calculated surplus of intake over energetic requirements without increase in the feeding rate. Lactation (and post-partem pregnancies) on the other hand, require the female to increase food intake during the summer months by up to 80 %. This appears to be achieved by increasing the period of foraging each day by about 5 h and selecting prey of much larger than average individual length and mass. Even this level of food intake is far less than maxima recorded in the literature for captive animals.