Estrogen and xenoestrogens (i.e. agents that are not steroids but possess estrogenic activity) increase the open probability (Po) of large conductance Ca2+-activated K+ (BK) channels in smooth muscle. The mechanism of action may involve the regulatory β1 subunit. We used β1 subunit knockout (β1-/-) mice to test the hypothesis that the regulatory β1 subunit is essential for the activation of BK channels by tamoxifen, 4-OH tamoxifen (a major biologically active metabolite), and 17β-estradiol in native myocytes. Patch clamp recordings demonstrate BK channels from β1-/- mice were similar to wild type with the exception of markedly reduced Ca2+/voltage sensitivity and faster activation kinetics. In wild type myocytes, (xeno)estrogens increased NPo (Po × the number of channels, N), shifted the voltage of half-activation (V1/2) to more negative potentials, and decreased unitary conductance. These effects were non-genomic and direct, because they were rapid, reversible, and observed in cell-free patches. None of the (xeno)estrogens increased the NPo of BK channels from β1-/- mice, but all three agents decreased single channel conductance. Thus, (xeno)estrogens increase BK NPo through a mechanism involving the β1 subunit. The decrease in conductance did not require the β1 subunit and probably reflects an interaction with the pore-forming α subunit. We demonstrate regulation of smooth muscle BK channels by physiological (steroid hormones) and pharmacological (chemotherapeutic) agents and reveal the critical role of the β1 subunit in these responses in native myocytes.