Plasma levels of LH, testosterone, estradiol, and progesterone were measured, during the breeding season, in adult and immature macaroni (Eudyptes chrysolophus) and gentoo (Pygoscelis papua) penguins, at Bird Island, South Georgia (54°S, 38°W), in relation to deferred sexual maturity. Macaroni penguins do not breed until 6–8 years of age whereas gentoo penguins first breed at 2 years of age. Plasma LH was elevated in 3- to 5-year-old macaroni penguins, but not in 1- to 2-year-old birds nor 1-year-old gentoos. However, 1 year olds of both species responded to an injection of exogenous LH-RH by increasing LH secretion. Some individual 3-, 4-, and 5-year-old macaroni penguins had circulating testosterone levels within the range of peak values in adults, suggesting that in males, gonads were physiologically mature several years in advance of the age of first breeding. One-year-old macaroni penguins had elevated progesterone levels and basal estradiol levels, but in 2–4 year olds plasma levels of both progesterone and estradiol were low. This does not support the hypothesis that high progesterone levels “block” ovarian maturation in immature penguins. Immature gentoo penguins showed a different pattern of hormone secretion. Plasma testosterone was low in all breeding periods, but estradiol levels increased to a peak during the incubation period, 4 weeks later than peak values occurred in adults. In both species, therefore, deferred sexual maturity was associated with delayed and/or reduced secretion of LH and gonadal steroids.