Body masses of over 1,500 adult and sub-adult Black-legged Kittiwakes Rissa tridactyla were measured during breeding seasons from 1954 to 1995 at colonies in NE England. There was little change between years in the mean mass of each sex, except for a 2% increase in the 1980s and a decrease of 4–7% in 1995. The relative constancy of mass, together with high breeding success throughout, suggested that periods of food shortage in the breeding season were absent throughout the study period. Breeding males averaged 394g and were about 13% heavier than females (340g). Breeding females showed a brief and temporary increase in mass prior to egg laying, and both sexes showed a small but non-significant increase in mass during incubation. There was an abrupt loss of about 6% of mass in adults of both sexes at the time the eggs hatched, and mass remained at this lower level through the nestling period. Masses of prospecting and breeding females were similar and both showed the abrupt decrease in mass in late May and in June. In contrast, prospecting males had a consistently lower mass (377g) than breeding males throughout. The pattern and timing of the loss of mass in breeding kittiwakes did not support the hypothesis that it is induced by stress arising from the need to obtain extra food for the brood, but that it can be regarded as an adaptation for more energetically efficient flight during a period when increased flying activity is needed to obtain additional food for the brood. The assumption that a higher mass in an individual is always a useful measure of quality is probably unjustified.