The regression of oestradiol-induced pituitary tumours in the rat.

Wistar rats of both sexes were implanted subcutaneously with oestradiol pellets (25 mg). The pellets were removed in two groups of animals after 120 and 220 days of implantation; in other animals the pellets were left for 230 and 370 days. The animals in whom pellets were left implanted all had large pituitary tumours. The animals from whom the pellets had been removed had pituitaries larger than normal, but much smaller than those in which the implants had been left. The results suggest that oestradiol-induced pituitary tumours are totally hormone-dependent and are not autonomous neoplasms. Oestradiol-induced testicular atrophy was also reversible.