Previous reviews have recognized patterns of lactation in pinnipeds divided along phylogenetic lines. This study extended previous models of lactation in pinnipeds by explicitly taking into account all the energetic costs to mothers. Based on an analysis of time-energy budgets, the feasible lactation strategy for a species can be shown to depend on body mass. Due to increased metabolic costs of maintenance, species with a large body mass cannot normally sustain lactation by foraging during lactation unless they have access to rich local prey resources. Consequently, large pinnipeds must normally sustain lactation from body reserves. This disadvantage is compensated in large pinnipeds by freedom to forage in support of offspring at greater range whereas small pinnipeds are restricted to foraging within the locality of the pupping colony. In the absence of correlations between major life-history variables and body mass in pinnipeds, the principal patterns of lactation are likely to be different solutions to the trade-off between foraging on a relatively rich prey resource at long range and foraging on a poorer prey resource within a restricted range. Hence phylogeny may be less important than adaptation in the evolution of pinniped lactation.