The combined effect of oestrogen and progesterone on the release of LH has been evaluated in female rats castrated three weeks earlier. In addition to progesterone, its 5α-reduced metabolites (5α-pregnan-3,20-dione, DHP; 5α-pregnan-3α-ol-20-one, 3α- ol; 5α-pregnan-3β-ol-20-one, 3β- ol, that are known to be formed in the brain and in the anterior pituitary, have also been studied. Two different approaches have been used: (1) Ethinyl oestradiol (EE) has been implanted into the median eminence (ME) of the animals, and left in situ for 5 days; the different progestagens were injected subcutaneously in a dose of 100 μg/rat in the afternoon of the 5th post-implantation day. The animals were killed 16 h later. Half of the progestagen-injected animals received 10 ng of LH-RH i.v. 20 min before sacrifice; 1. (2) EE has been placed directly into the anterior pituitary (AP) and again the implant has been followed by systemic treatment with the various progestagens. In these animals, the hypothalamic LH-RH content has been measured in addition to serum levels of LH. The results indicate that: 2. (3) EE implants either in ME or in AP significantly depress serum LH; 3. (4) In ME-EE-implanted animals, either P or DHP cause a large increase in serum LH, while neither 3α-ol nor 3β-ol are effective; on the contrary in the animals bearing AP implants no stimulatory effect was induced by progestagen administration; 4. (5) The pituitary of ME-EE implanted animals is much more responsive to LH-RH than that of sham-implanted controls, as indicated by the increase in serum LH after LH-RH administration; 5. (6) P and DHP significantly reduce LH-RH responses in ME-EE implanted animals; on the contrary 3α-ol and 3β-ol dramatically increase the pituitary responsiveness to the decapeptide in the same experimental conditions; 6. (7) Castration significantly reduces LH-RH content in the hypothalamus; AP-implanted oestrogens either alone or in combination with P or DHP are unable to re-establish normal intrahypothalamic stores of LH-RH; on the other hand 3α-ol givel to AP-EE-implanted animals lowers hypothalamic content of the decapeptide, while increasing its serum levels.