If harbor porpoises (Phocoena phocoena) are impaired in their foraging ability because they need to move away from anthropogenic sound sources, their fitness may be reduced. Understanding how much harbor porpoises can eat after a disturbance, and how quickly they can replenish their energy reserves, is important for assessing the significance of disturbances. After fasting for various time periods (2 to 24 h), four captive harbor porpoises, housed in water and air temperatures similar to those encountered by wild conspecifics, were fed a structured diet of meals larger than usual (each normal meal was 20% of the daily food mass requirement). A few times they were fed ad libitum, but this led to severe constipation, so this feeding method was abandoned for welfare and health reasons. The food ingested over a period of one hour following fasting for 2 to 24 hours was quantified (i.e., mass, volume, and as a percentage of normal daily food mass intake in that period). The results show that, in contrast to established belief, harbor porpoises can eat a large percentage (up to ~98%) of their normal daily food mass intake in a single feeding bout without showing physical problems. Adult animals of around 155 cm in body length can eat up to ~3 kg (~2,700 ml) in one feed. If food is abundantly available after a period of fasting due to a disturbance, wild harbor porpoises could eat a large percentage of their daily energetic requirement in one feeding bout to compensate for the period of fasting. However, if food availability is limited in terms of prey numbers, size, or species, or if the fish are widely dispersed (so that more time is required to find and capture them), this may limit or reduce the speed of the recovery of body mass and blubber layer.