The objective of the present study was to determine how rapidly estradiol (E₂) was able to suppress the secretion of LH in ovariectomized (OVX) ewes and to evaluate the ability of conjugated forms of E₂ (E₂ conjugated to BSA [1,3,5(10)-estratrien-3,17β-diol-6-one-6-carboxymethyloxime:BSA [E₂-BSA] and a novel conjugate, E₂ conjugated to a small peptide [E₂-PEP]) to mimic the actions of E₂ on secretion of LH and FSH. Animals (n = 5–6 per group) were given infusions for 4 h of 50 μg of E₂ or equimolar concentrations of E₂-BSA or E₂-PEP. Treatments with E₂, E₂-BSA, and E₂-PEP each induced an acute suppression of LH secretion (<20 min, P < 0.01). In contrast, E₂, but not E₂-BSA or E₂-PEP, induced the characteristic preovulatory-like surge of LH (at 10 h after priming treatment) and decreased secretion of FSH (at 4 h after priming treatment). In conclusion, the acute inhibition of LH secretion induced by E₂ in OVX ewes supports the concept of a nongenomic action as the mechanism underlying the sudden suppression in secretion of LH. In addition, the fact that conjugated forms of E₂ mimicked only the acute suppression of secretion of LH, without inducing the putative genomic actions on secretion of LH or FSH (i.e., a preovulatory-like surge), suggests that the acute effect of E₂ may be mediated via the plasma membrane.