Plasma prolactin levels were measured in free-living breeding adult and immature (prebreeding) macaroni (Eudyptes chrysolophus) and gentoo (Pygoscelis papua) penguins at Bird Island, South Georgia (54°S, 38°W). Macaroni and gentoo penguins first breed at 5-6 and 2 years of age, respectively. In adult birds, of both species, prolactin was low (<1.0 μg.liter-1) during the courtship period and then increased during early (gentoo) to mid (macaroni) incubation (to 3.9-4.7 and 2.4-3.5 μg.liter-1, respectively), remaining elevated until the creche period, by which time continuous nest attendance by the adults had ceased. This pattern is similar to that seen in other altricial species and is consistent with delayed onset of brood patch development and full incubation efficiency, which has been previously reported in penguins. Adult female macaroni penguins showed a marked, but transient, increase in prolactin concentrations within 24 hr of the first egg being laid (from 1.7 to 7.0 μg.liter-1), plasma levels decreasing following clutch completion (to prelaying levels) before increasing again during incubation. Elevated plasma prolactin levels occurred in all age classes of immature (nonbreeding) birds in both macaroni (1- to 5 year-olds) and gentoo (1-year-olds) penguins. However, compared to that in adult birds, the increase in prolactin was more transient in immatures, a smaller proportion of immatures had detectable prolactin levels at each stage of the breeding cycle, and, at least in 1- and 2-year-olds, absolute levels of prolactin were lower. It is suggested that variation in the pattern of prolactin secretion in immature birds represents real, age-related or species-related differences and does not simply reflect differences in the previous photoperiodic environment experienced. The physiological significance of prolactin secretion in immature (nonbreeding) birds remains unknown, but it may simply reflect the existence of a photoperiodically driven prolactin release system which does not become physiologically important until the bird becomes sexually mature and breeds.