Primary pituitary cell cultures from adult female rats were incubated for 4 or 24 h with various concentrations of estradiol (E₂) or the antiestrogens (AE) tamoxifen (TMX), 4-hydroxytamoxifen (OH-TMX), and clomiphene citrate (CC). The luteinizing hormone (LH)-response of these cultures to gonadotrophin releasing hormone (GnRH) was monitored. Treatment for 4 or 24h with high concentrations (10^–5M) of the AE significantly decreased the GnRH-induced LH-release by the gonadotrophs. The negative E₂-effect, which is observed in this model after 4h was enhanced and the positive E₂-effect, which occurs after 24h, was completely reversed into a negative effect by these high AE-concentrations. Treatment of pituitary cells with increasing concentrations of E₂ (10^–13–10^–6M) or AE (10^–12–10^–5M) for 24h led first to a dose dependent increase of the LH-response to 5 × 10^–10M GnRH. At higher E₂- or AE-concentrations this positive effect was lost, resulting in bell shaped dose-response curves. The following maximal effective concentrations (ED_max) were found: E₂ = 10^–10M, OH-TMX = 10^–9M, CC = 10^–7M, TMX = 10^–6M. Incubation of pituitary cells for 24h with concentrations of AE near their ED_max and stimulation with increasing concentrations (10^–11–10^–7M) of GnRH resulted in significant increases of LH-secretion over a wide range of GnRH-concentrations. It is concluded that AE possess marked intrinsic activities on pituitary LH-secretion: at extremely high concentrations they suppress the GnRH-induced release of the gonadotrophin. At lower concentrations they increase the pituitary LH-response to GnRH in a manner which is qualitatively indistinguishable from that of E₂.