SUMMARY In the course of a 10-year study (1936-1946) of the Yellow-eyed Penguin, Megadyptes antipodes, weights were taken at 7-day intervals from 56 days before the egg was laid to 48 days after hatching-a period of about 146 days. Males reach their peak weight at the end of the off-season and drop steadily during the pre-egg stage until just before egg-laying. Females reach their peak in weight a week later than the males, and for four weeks are heavier than the males, which again exceed the females in weight as the eggs appear. As incubation progresses, both sexes steadily increase in weight, but another drop occurs during the 48 days after the hatching of the chick. Weight fluctuations in two species of petrel, the Broad-billed Prion, Pachyptila vittata [=forsteri], and the Diving Petrel, Pelecanoides urinatrix, were also examined. The same pattern of fluctuation seemed to occur in these two species as in the penguin Megadyptes. The results of the study were compared with the findings of other workers on other species. They were found to follow closely the deductions of Wolfson, who studied migrant Oregon Juncos, Junco oreganus. They differ, however, from data on non-migratory birds, most of which apparently reach peak weights in the winter. Megadyptes resembles Juncos in that a heavy deposit of fat develops as the pre-egg period approaches. In both species this is coincident with growth of the gonads. The pattern of weight fluctuation indicates that incubation in birds is not the arduous task that it is supposed to be, but that feeding of young is a time of stress. The penguin Megadyptes reaches its peak weight for the year at the beginning of the molt, its minimum weight at the end; the petrel Pachyptila apparently does not lose weight during the molt. Evidence from other workers on other species indicates that in some birds there is a gain of weight during the molt; in others, a loss; in still others, no significant change.