In the golden hamster, there are marked sex differences in the Harderian gland. Male glands (which are heavier than female glands) possess two cell forms (Type I and Type II cells); female glands only exhibit the former. Female (but not male) glands contain large amounts of porphyrin, which are readily visible as solid depositions within the lumina. The weight, histology and porphyrin content of the Harderian gland was examined in intact adult male hamsters and in male hamsters castrated for 1,2 or 8 months. Castration resulted in a significant reduction in the weight of the gland, the disappearance of Type II cells, and the presence in the gland of solid porphyrin accretions. The levels of copro- and (especially) protoporphyrin were greatly increased. These changes were more marked with time after castration. When the ability of diverse androgens (testosterone, 5alpha-dihydrotestosterone, androst-4-ene-3,17-dione (androstenedione), dehydroepiandrosterone and androsterone) to prevent these changes was tested, testosterone and androstenedione maintained glandular weight. All the androgens maintained normal frequencies of Type II cells and all except dehydroepiandrosterone prevented deposition of porphyrin. The potencies of the various androgens in maintaining normal Harderian gland morphology and activity are compared with their effects on other somatic variables and sexual behaviour.