Background: The small conductance Ca2+-activated K+ current (ISK) may change the cardiac atrial action potential shape in response to altered intracellular [Ca2+], and thus may be a potential therapeutic target for treating atrial fibrillation (AF). Aim: To measure [Ca2+]i-sensitivity of atrial ISK. Methods: Myocytes were enzymatically isolated from atrial tissues from patients undergoing elective cardiac surgery and from rabbit hearts. Whole-cell-patch clamp, with voltage ramps (-120 to +50 mV in 7 s), varying [Ca2+]i (~100-500 nM; set using 5 mM BAPTA, verified using Fura-2), and conventional (apamin) or putative selective (ICA) ISK blockers. Results: A +ve control was required since ISK would be difficult to detect (low density): we tested temporal stability, timing, and reversibility of K+ current (IK1) block by Ba2+, in rabbit. Ba2+ (0.5 mM) significantly decreased inward IK1 (at -115 mV) in 15/16 (94%) left ventricular cells, from -38.9±5.9 to -12.9±4.5 pA/pF (by 67%), and in 23/25 (92%) left atrial cells, from -20.3±4.2 to -11.6±2.9 pA/pF (by 43%) (P<0.05 for each; t-test, n=11-16 rabbits). Ba2+ effect onset-time was 30-60 s. All responses were then stable, and reversible in 88% of 24 cells. Ba2+ also decreased outward IK1 (at -65 mV), e.g. in atrial cells: from +1.5±2.7 to -5.2±4.0 pA/pF (P<0.05). In rabbit atrial cells, with ~100 nM [Ca2+]i, apamin (100 nM) had no effect on inward current: -5.9±0.6 vs -6.3±0.6 pA/pF control (P=0.35), or outward current: +0.4±0.4 vs +0.7±0.3 pA/pF (P=0.35; 10c, 8r). With ~300 nM [Ca2+]i, apamin also had no effect on inward (-6.2±0.5 vs -6.5±0.6 pA/pF) or outward current (+0.6±0.3 vs +0.8±0.3 pA/pF; 26c, 11r). With ~500 nM [Ca2+]i, apamin again showed no effect (-15.4 ± 4.3 vs -14.8 ± 4.0 pA/pF, and +2.2±0.7 vs +2.2 ±0.7 pA/pF; 10c, 7r). Furthermore, ICA (1 µM) also had no effect on rabbit inward or outward currents at any [Ca2+]i. In human right atrial cells, ICA was tested at [Ca2+]i of ~100 nM (5 cells, 3 patients), ~300 nM (10c, 4p) and ~500 nM (8c, 4p); apamin at [Ca2+]i ~300 nM (7c, 3p). Neither apamin nor ICA significantly affected inward or outward current at any [Ca2+]i. Considering that ISK might occur in only a small proportion of myocytes, potentially masked by cell-averaging, data were checked for any individual cell ISK-blocker effect, of any magnitude, compatible with the +ve control onset/reversibility profile: a “candidate drug effect” (CDE). Of 84 cells studied from either species/drug, 5 CDEs (6% of cells; 1.2±0.3 pA/pF) occurred, whether or not BSA (to avoid apamin adherence) or BDM (to inhibit cell contracture) were used. CDEs occurred at [Ca2+]i of ~300 or ~500 nM, and not ~100 nM; but only with apamin, and were non-reversible. Conclusion: ISK is either non-existent or rare, in rabbit or human atrial cardiomyocytes, at [Ca2+]i typical of global diastolic-to-systolic values.