The effects of hypothalamic implants of ovarian steroids on oestrous behaviour in rabbits.

1. The brains of ovariectomized female rabbits which were anoestrous in behaviour when tested with vigorous males were implanted either bilaterally or unilaterally with 10% oestradiol benzoate in cholestrol.2. Rabbits with implants in the ventromedial-premammillary hypothalamus ca. 1 mm lateral to the mid line became highly oestrous in behaviour within 1-2 days of implantation and mated 100% of the times tested. Implants of blank tubing, cholesterol or progesterone in this area, or oestrogen implants elsewhere in the hypothalamus had no effect on sexual behaviour.3. The uteri of all females implanted with diluted oestrogen were atrophic and similar to those of castrate controls, indicating that the effects of hypothalamic implants on sexual behaviour were direct local effects of oestrogen on the C.N.S.4. Rabbits implanted bilaterally with undiluted oestradiol benzoate showed some slight systemic spread of the implanted oestrogen. Oestrous behaviour was evoked within an average of 3.4 days by implantation into a large hypothalamic area, presumably as a result of oestrogen diffusing from the site of implantation to the critical ventromedial-premammillary region.5. Injections of progesterone in dosages known to block oestrous behaviour in intact or oestrogen-primed rabbits, failed to block this behaviour in females implanted with pure oestrogen in the ventromedial-premammillary hypothalamus.6. Implantation of progestins into the ventromedial-premammillary area in overiectomized rabbits did not usually inhibit oestrous behaviour induced by small threshold doses of systemically administered oestrogen. The possibilities that oestrogen and progesterone (1) interact in different areas of the C.N.S. or (2) compete for ;steroid receptors' in the ventromedial-premammillary hypothalamus are discussed.