Aim: To investigate the effects of docosahexaenoic acid (DHA) on large-conductance Ca2+-activated K+ (BKCa) channels and voltage-dependent K+ (K-V) channels in rat coronary artery smooth muscle cells (CASMCs). Methods: Rat CASMCs were isolated by an enzyme digestion method. BKCa and K-V currents in individual CASMCs were recorded by the patch-clamp technique in a whole-cell configuration at room temperature. Effects of DHA on BKCa and K-V channels were observed when it was applied at 10, 20, 30, 40, 50, 60, 70, and 80 mu mol/L. Results: When DHA concentrations were greater than 10 mu mol/L, BKCa currents increased in a dose-dependent manner. At a testing potential of +80 mV, 6.1%+/- 0.3%, 76.5%+/- 3.8%, 120.6%+/- 5.5%, 248.0%+/- 12.3%, 348.7%+/- 17.3%, 374.2%+/- 18.7%, 432.2%+/- 21.6%, and 443.1%+/- 22.1% of BKCa currents were increased at the above concentrations, respectively. The half-effective concentration (EC50) of DHA on BKCa currents was 37.53 +/- 1.65 mu mol/L. When DHA concentrations were greater than 20 mu mol/L, K-V currents were gradually blocked by increasing concentrations of DHA. At a testing potential of +50 mV, 0.40%+/- 0.02%, 1.37%+/- 0.06%, 11.80%+/- 0.59%, 26.50%+/- 1.75%, 56.50%+/- 2.89%, 73.30%+/- 3.66%, 79.70%+/- 3.94%, and 78.1%+/- 3.91% of K-V currents were blocked at the different concentrations listed above, respectively. The EC50 of DHA on K-V currents was 44.20 +/- 0.63 mu mol/L. Conclusions: DHA can activate BKCa channels and block K-V channels in rat CASMCs, and the EC50 of DHA for BKCa channels is lower than that for K-V channels; these findings indicate that the vasorelaxation effects of DHA on vascular smooth muscle cells are mainly due to its activation of BKCa channels.