Alterations to cardiac gap-junction coupling and consequent changes to action potential (AP) propagation may predispose the heart to arrhythmias. Known cellular uncoupling agents may also alter membrane ionic currents and hence AP parameters, complicating studies of the their action. We aimed to compare the effects of carbenoxolone and n-octanol on AP morphology and conduction velocity (θ) in guinea-pig left ventricular myocardium. Male Dunkin-Hartley guinea pigs were sacrificed after schedule 1 methods, the heart was removed and left ventricular papillary muscles superfused in Tyrodes solution at 37 C. Longitudinal θ and AP parameters were measured before, during and after addition of either 20μM carbenoxolone or octanol 0.096ml/ml in Tyrodes solution for up to 60 minutes. Data are mean ±SD, and sets were compared with Students t-test, significance at p<0.05. 20μM carbenoxolone had no effect on AP parameters (control first): i) action potential duration at 50% repolarisation, APD₅₀; 154±12 vs 155±16 ms, n=6: ii) APD₉₅; 193±17 vs 205±17 ms, n=6: iii) maximum AP upstroke velocity, V_max; 215.9±44.9 vs 195.6±46.2 V/s, n=6: iv) the time constant of the AP foot, τ_ap,; 0.28±0.04 vs 0.28±0.02 ms, n=5. However, carbenoxolone significantly slowed θ, from 76.5±5.3 to 39.8±5.2 cm.s-1 (n=5) after 45 minutes. R_i, derived from the cable equation θ² = a/2R_i.C_m.τ_ap (a is cell radius, and C_m = 1 μF/cm²) increased from 395±28 to 1528±314 Ω.cm. The effects of carbenoxolone were irreversible at this concentration. n-Octanol also significantly slowed θ by 29.6±14.9% (n=5) after 7 minutes, and further by 65.8±26.8% (n=3) after 30 minutes. Conduction was completely blocked within 10 minutes in two of five preparations. APD₅₀ decreased from 163±15 to 101±16 ms (n=5) after 7 minutes and to 96±15 ms (n=3) after 30 minutes. APD₉₅ decreased from 209±11 to 141±18 ms after 3 minutes, and to 127±14 ms after 30 minutes. V_max and τ_ap were unaffected. After superfusion with Tyrodes for 60 minutes θ and AP parameters returned to normal in all preparations. Carbenoxolone slowed ventricular AP conduction by increasing R_i, and without altering AP morphology, whereas n-octanol also significantly reduced APD. These results suggest that carbenoxolone, but not n-octanol, specifically uncouples gap-junctions without effect on ionic currents.