The present study examined the dose- and time-dependent central effects of an estradiol chemical delivery system cyclodextrin complex (E(2)-CDS-CD) on the reestablishment of copulatory behavior of castrated male and ovariectomized female rats with concomitant determination of the blood luteinizing hormone (LH) and E(2) levels. In orchidectomized males, Estredox, after single doses of 0.3 and 3.0 mg/kg iv, reestablished the mounting and intromission up to 4 weeks. The LH suppressive effect lasted to Day 7 and 28, respectively. After repeated administration for 10 days at a dose of 0.01mg/kg iv, significant effect was obtained by Day 14. Ovariectomized females were treated iv daily for 5 days either with E(2)-CDS-CD, estradiol benzoate (EB) or vehicle, and the lordosis quotient was determined. At a dose of 0.03 mg/kg the duration of EB's effect was 10 days shorter and only one-third of that of E(2)-CDS-CD. The LH suppression lasted to Day 18. On the other hand, after EB treatment there was no significant decrease in LH levels. The low plasma E(2) levels indicated fast rate of peripheral elimination in both males and females. The brain-targeting E(2) indicates better efficacy and increased safety in replacement therapies because of the reduced peripheral side effects.