There are marked sex differences in the Harderian gland of the C3H/He strain of mice. Female (but not male) glands contain large amounts of porphyrin, which are readily visible as solid depositions within the lumina. The histology and porphyrin content of the Harderian gland were examined in intact and in pregnant mice and in mice subjected to combinations of adrenalectomy, gonadectomy and administration of sex steroid hormones. In male mice, castration approximately doubled the amount of porphyrin in the Harderian gland. Castration plus adrenalectomy increased the levels over 30-fold, to levels similar to those found in female mice, although adrenalectomy alone produced no significant effect. Administration of testosterone to the male mice which had been castrated and adrenalectomized prevented the increases while progesterone treatment produced further increases in porphyrins. In intact females, the amount of porphyrin varied with the phase of the oestrous cycle; being lowest during metoestrus and highest during dioestrus. In ovariectomized-adrenalectomized females, the effects of administered sex hormones on the amount of porphyrin in the gland were the same as in males. In pregnant mice, the level was no significantly different from that in intact oestrous animals.