Stomach-content analysis can be used to identify the species and size of prey fishes eaten by marine mammals, and has been used for both qualitative and quantitative estimates of their food requirements. Methods that use mass or volume to determine the amount of fish eaten depend on the degree of digestion and the time required for the stomach to empty. The transit time of digesta was estimated by recording the initial defecation time and the time required for the stomach to empty in four harbour seals (Phoca vitulina). The seals were given meals varying in size from 0.5 to 2.1 kg and in caloric content from 4940 to 12 770 kJ/kg, and containing carmine or barium sulphate as a marker. The time from eating until red faeces were observed was between 2 h 35 min and 6 h 15 min. Also, barium sulphate was injected into the food given, and X-rays were used to estimate the time required for the stomach to empty. The stomach started to empty less than 1 h after the meal. Some prey remained in the stomach 5 h after eating.