The analysis of life-history traits suggests that the age and size of female mammals at parturition will affect the birth size and survival of their offspring. We collected data from 252 mother–pup pairs of northern fur seals on Bering Island, Russia, during the 1994 and 1996 breeding seasons to determine the interrelationships among the mothers' age and mass, the sex of the pup, the parturition date, and the length and mass of the pup at birth. Among reproductive females of 19 years, mass increased with age. The mean mass of the oldest class of females (20–23 years) was lighter than the 16–19 year age group but not different from the mean mass of the 7- to 15-year-old females; a quadratic model of mass on age gave similar results, and indicated a maximum mass at age 19 years. We suggest that this may be an effect of a higher rate of survival among leaner animals. Female pups were lighter than males at birth. There was no evidence that the sex ratio of pups differed from 1:1 over the range of observed mothers' mass and age or parturition dates. Older mothers tended to give birth earlier than younger mothers, and heavier mothers earlier than lighter mothers. The relationship of the size of pups at birth and their mothers' age was adequately described with a quadratic model, which predicted a maximum size for mothers at age 12–13 years. The size of pups at birth and the size of their mothers was described with a logistic model which predicted that the size of pups increased for mothers up to 41 kg, with no further increase for heavier mothers. The total amount of variability in birth size explained by the combined models is < one-third of the total, which implies that other influences, such as the contribution of the fathers and individual variation, are also important. The relative maternal investment, measured as the ratio of pup mass to mothers' mass, ranged from 20% for younger and smaller females to 10% for older and heavier females. In addition, relative maternal investment was found to be higher than for other pinnipeds.