We measured plasma levels of progesterone and prolactin in free-living adult breeders, adult-plumaged helpers, and juvenal-plumaged helpers of the cooperatively breed- ing Harris' Hawk, Parabuteo unicinctus, at different stages of the nesting cycle. Overall, pro- gesterone levels did not vary significantly with sex, nest stage, or behavioral role, but they increased significantly with handling time. Progesterone levels were highest (2-4 ng/ml) in breeding females just before egg laying. Prolactin levels did not vary with handling time but were significantly elevated in breeding females during the nest-building stage (14.1 ng/ ml) and in both breeding males (8.4 ng/ml) and females (25.5 ng/ml) during incubation. Both sexes of breeders incubate the eggs, but the females do the bulk of incubation and only they develop a brood patch. Prolactin levels in the breeders declined immediately after the eggs hatched, in contrast to the pattern seen in many other altricial species. Prolactin levels in the adult-plumaged male helpers rose significantly after the eggs hatched (9.1 ng/ml). At this time these birds bring more food items from group kills to the nestlings than any other group members. The elevated prolactin levels in the adult-plumaged helpers may facilitate the helping behavior that they exhibit toward the nestlings and fledglings.