Rationale: Ca2+ waves in cardiac myocytes lead to arrhythmias by inducing after-depolarisations. Waves occur when sarcoplasmic reticulum (SR) content reaches a threshold level. Phosphodiesterase-5 inhibitor sildenafil is antiarrhythmic in mammalian models of myocardial ischaemia via an unknown mechanism. Objective: Experiments in sheep ventricular myocytes were performed under voltage clamp and intracellular Ca2+ measured using Fura-2. Ca2+ waves were induced by elevating external Ca2+ (10-15mmol.l-1). To determine ‘threshold’ SR content, caffeine (10mM) was added immediately following a wave, and both wave and caffeine-induced INCX integrated. Sildenafil (1µM) abolished waves in 9/15 cells by reducing SR content below threshold. Sildenafil treatment reduced rate constant of SERCA (kSERCA -66% of control, p<0.005), transiently increased sarcolemmal efflux via INCX tail current (INCX tail high Ca2+ 4.99+/-1.0 µmol.l-1 vs first 4s Sil high Ca2+ 8.51+/-1.43 µmol.l-1 , p<0.01, n=20 cells / 14 animals), and reduced sarcolemmal influx via ICa-L (integrated ICa-L 4.12 µmolL-1 high Ca2+ vs 3.87 high Ca2+ Sil, p<0.0001). Where cells continued to wave in sildenafil, threshold was unchanged (118.9+/-16.8 µmol.l-1 ctrl high Ca2+ vs 137.4+/-29.2 µmol.l-1 Sil+high Ca2+, p=0.6, n=8-14 cells, 9 animals), and waves occurred later within each cycle. In separate experiments on waving cells, inhibition of SERCA with thapsigargin (5µmol.l-1) mimicked the sildenafil effect on waves. Sildenafil protective effects on waves were abolished in cells preincubated with PKG inhibition (KT5823, 1µmol.l-1). Conclusions: Sildenafil abolished Ca2+ waves via a PKG-dependent mechanism. Suppression of waves is mediated by reduced SR content, itself caused by reduced SERCA function +/- reduced ICaL. These findings highlight novel antiarrhythmic properties of PDE5 inhibitors.