Mammalian BK channels are modulated by estrogen and non-steroidal estrogen-like compounds (i.e. xenoestrogens), but the effects are dependent on channel composition. (Xeno)estrogens preferentially activate BK channels through accessory beta subunits, but reduce single-channel conductance by interaction with alpha subunits. In this report, the xenoestrogen tamoxifen was applied to chicken BK channels, in order to asses the mechanism behind drug interaction and to determine the extent to which (xeno)estrogen interaction is extended to avian BK homologs. As with mammalian isoforms, the properties of chicken BK channels were modulated by tamoxifen in a subunit-dependent manner. Tamoxifen reduced single-channel conductance through interaction with the alpha subunit. However, if the expression construct included the beta subunit, tamoxifen increased the channel's open probability and shifted the voltage-activation range to more negative potentials. This effect on channel gating was concentration-dependent, with an EC(50) of about 0.2 microM. Tamoxifen-mediated reductions in gating charge and in the intrinsic energetics that govern channel equilibrium. The relative contribution of these two effects on channel gating was altered by beta co-expression. Modulation by (xeno)estrogens may be an evolutionarily conserved mechanism for non-genomic hormonal actions, and the limited conservation between avian and mammalian beta subunits may suggest potential binding motifs. Alternatively, the data are consistent with a tamoxifen-mediated conformation change in the alpha subunit that alters the way alpha and beta subunits interact, resulting in enhanced gating without direct binding to beta.