Objective: We showed recently that the 825T allele of the G-protein β3-subunit C825T polymorphism is associated with large inward rectifier K⁺ currents I_K1 but low acetylcholine-activated K⁺ current I_K,ACh amplitudes. During chronic atrial fibrillation (AF), I_K1 and I_K,ACh current densities were increased when compared to sinus rhythm (SR). It is unknown whether chronic AF and Gβ3 gene status are independent contributors to atrial K⁺ current activity. We measured I_K1 and I_K,ACh in tissue from AF patients with different Gβ3 genotypes and assessed the relation between the I_K1 and I_K,ACh amplitudes and the incidence of postoperative AF. Methods: We measured the amplitudes of I_K1 and I_K,ACh in atrial myocytes from 26 patients with sinus rhythm (SR) and from 16 patients with chronic AF (<6 months). The K⁺ currents were measured with standard patch-clamp techniques. The Gβ3 gene status of the patients was determined by PCR and restriction analysis. Results: At −100 mV, the amplitude of I_K1 was larger in AF (10.9±1.0 pA/pF, n=49/16, cells/patients) than in SR (6.3±0.6 pA/pF, n=68/26, P<0.05), whereas the amplitude of I_K,ACh was smaller in chronic AF (2.9±0.7 pA/pF, n=49/16) than in SR (6.3±0.7 pA/pF, n=68/26, P<0.05). These changes were independent of the patient Gβ3 gene status. Eight patients out of 26 in the SR group (31%) developed postoperative AF. When analysed based on incidence of postoperative AF, current amplitudes did not differ significantly. Conclusion: We provide evidence for up-regulation of I_K1 but down-regulation of I_K,ACh in chronic AF which are independent of Gβ3 gene status. Atrial myocytes from patients who are in SR but later develop postoperative AF have no manifestation of altered I_K1 and I_K,ACh at the time of cardiac surgery. Our results suggest that the AF-related changes of I_K1 and I_K,ACh may be a consequence of or a contributory factor to chronic AF.