Estrogens play essential roles in the neuroendocrine control of reproduction. In the present study, we focused on the effects of 17-estradiol (E2) on the K⁺ currents that regulate neuronal cell excitability and carried out perforated patch-clamp experiments with the gonadotropin-releasing hormone (GnRH)-secreting neuronal cell line, GT1–7. We revealed that a 3-day incubation with E2 at physiological concentrations (100 pM - 1 nM) augmented Ca²⁺-activated K⁺ (K(Ca)) currents without influencing Ca²⁺-insensitive voltage-gated K⁺ currents in GT1–7 cells. Acute application of E2 (1 nM) had no effect on the either type of K⁺ current. The augmentation was completely blocked by an estrogen receptor (ER) antagonist, ICI-182,780. An ER-selective agonist, DPN, augmented the K(Ca) currents, although an ER-selective agonist, PPT, had no effect. Knockdown of ER by means of RNA interference blocked the effect of E2 on the K(Ca) currents. Furthermore, semi-quantitative RT-PCR analysis revealed that the levels of BK channel subunit mRNAs for and 4 were significantly increased by incubating cells with 300 pM E2 for 3 days. In conclusion, E2 at physiological concentrations augments K(Ca) currents through ER in the GT1–7 GnRH neuronal cell line and increases the expression of the BK channel subunit mRNAs, and 4.