1. Adult female harbour seals from NE Scotland were radio-tracked to follow changes in their distribution and activity during the breeding season. 2. Seals foraged up to 45 km from haul-out sites, but females with pups restricted their range markedly during the early part of the lactation period. However, foraging trips resumed before the expected weaning date, indicating that female harbour seals do not fast throughout lactation. 3. The duration of the period that females spent inshore with young pups was positively related to female body size, suggesting that the species' small size may demand that they feed to maintain lactation. 4. Foraging in late lactation results in females becoming highly dispersed during the mating period and it is clearly uneconomic for males to monopolize females.