Changes in body mass and body reserves of Little Auks (Alle alle) were studied throughout the breeding season. Body mass loss after chick hatching was analyzed with respect to two hypotheses: (1) mass loss reflects the stress of reproduction, (2) mass loss is adaptive by reducing power consumption during flight. Body mass of both males and females increased during incubation, dropped abruptly after hatching, and remained stable until the end of the chick-rearing period. These changes were largely due to change in mass of fat reserves. Body mass, fat, and protein reserves, when corrected for body size, did not differ between sexes at the end of incubation. Female size-corrected body mass at that time was correlated with peak body mass of chicks. The estimated energy savings for flight due to the decline in adult body mass after chick hatching were small compared with the total energy expenditure of adults feedings chicks, which did not support hypothesis (2). The contribution to chick feeding was not equal; the ratio of females to males caught with food for chicks was 1.8. Size-corrected body mass during chick-rearing was lower in females, proportional to their higher chick feeding effort compared with males. Females, in contrast to males, lost protein reserves during chick-rearing. Digestive tract mass of adults increased by half throughout the breeding period. These findings supported elements of hypothesis (1). Despite high energy expenditure rates, both sexes had about 10 g of fat reserves at the end of chick feeding. Body mass of both sexes was constant during the greater part of the chick-feeding period. It was suggested therefore that mass loss is regulated with respect to lower fat reserves required during chick-rearing.