The hamster Harderian gland seems to present both an excellent model for the control of porphyrin biosynthesis and an unusually robust example of the interrelationship between structure and function. It has been known for some time that 1) the capacity for manufacturing and storing porphyrins and 2) gland histology and ultrastructure are controlled by androgens. Thus, in intact males as well as in gonadectomised animals of either sex treated with androgens, porphyrin synthesis by the Harderian gland is suppressed and the gland tubules characteristically possess two cell types, the cytoplasm of both containing polytubular complexes. By contrast, the Harderian glands of intact females and castrated males synthesise and store large amounts of protoporphyrin, while their tubules possess only one cell type which lacks a polytubular complexes. So overarching is the effect of androgens that they have been described as a "coarse tuning" effect on the gland. By contrast, the role of the ovary is both less dramatic and less well understood. In female hamsters, ovariectomy leads to degenerative changes in Harderian gland tubules and (probably) a release of stored porphyrin; at the same time there is a reduction in enzyme levels and new synthesis. The causative hormone in this "fine tuning" is unclear at present. There is now clear evidence that the Harderian gland is also controlled directly by pituitary hormones. In particular, the use of continuous infusion osmotic minipumps has allowed us to demonstrate not only 1) that the expected rise in porphyrins and feminisation of gland morphology does not occur in castrated males receiving the dopamine agonist bromocriptine, but that 2) the simultaneous administration of prolactin does permit these changes; furthermore, 3) the administration of prolactin alone increases porphyrin synthesis above the levels found in untreated castrates. Similarly, bromocriptine administration to ovariectomised females markedly reduces porphyrin synthesis and masculinises gland structure; again, this is reversed by the simultaneous administration of prolactin. Prolactin must therefore be seen as equipotent with androgens in determining gland structure and activity.