We have explored the role that inhibitory G-proteins have in ventricular arrhythmia. Mice with global deletion of Gαi2 were generated by homologous recombination on a 129SvEv background. Matched Gαi2 (+/+) were generated as littermates in Gαi2(+/-) crosses. Mice were injected with heparin (250 IU) and anaesthetised with a combination of ketamine 10 mg/ml, xylazine 2mg/ml, atropine 0.06 mg/ml; where a dose of 0.01 ml/mg was injected by perinetal route. The hearts were rapidly excised, cannulated and put on a Langendorff perfusion system. Single ventricular myocytes were obtained by enzymatic digestion. Data are compared with Student’s t-test. Gαi2 (-/-) mice have a prolonged QT interval on the surface ECG when studied with telemetry. Using in-vivo electrophysiology studies the Gαi2 (-/-) mice have a reduced ventricular effective refractory period and a predisposition to ventricular tachycardia when challenged with programmed electrical stimulation. The absence of Gαi2 thus leads to a primary electrical abnormality and we explored the basis for this. Using single cell electrophysiology, we found that action potential from isolated single cardiac ventricular myocytes showed a prolonged phase of repolarisation in Gαi2 (-/-) mice as reflected in a significantly prolonged APD50 (9.9 ± 1 ms, n=16 for Gαi2 (+/+) mice and 16.9 ± 2.2, n=11 for Gαi2 (-/-) mice, p<0.05). We next examined the rate dependence by constructing single cell restitution curves. There were significant differences between the Gαi2 (-/-) mice and Gαi2 (+/+) mice under basal conditions (p<0.01) and in the presence of isoprenaline (p<0.01). Specifically, the slope of the restitution curve in its linear phase was steeper in the Gαi2 (-/-) mice (linear regression with slopes were compared using analysis of covariance). Under basal conditions: Gαi2 (+/+) mice (20 to 120 ms) r2=0.88, slope=0.20±0.03; Gαi2 (-/-) mice (20 to 120 ms) r2=0.93, slope=0.43±0.04 (p<0.001). In the presence of isoprenaline: Gαi2 (+/+) mice (20 to 120 ms) r2=0.90, slope= 0.27±0.03; Gαi2 (-/-) mice (20 to 100 ms) r2=0.87, slope= 0.49± 0.07 (P=0.01). The observed abnormalities in repolarisation could be accounted for by increases in inward current in particular via L-type Ca2+ channels or a decrease in outward K+ currents. There was no significant differences in the K+ currents between the two groups. However, there was a significant increase in L-type Ca2+ currents in the Gαi2 (-/-) mice. The currents amounted -3.9 ± 1 pA/pF in Gαi2 (+/+) mice, and -7.8 ± 1.1 pA/pF in Gαi2 (-/-) mice (p<0.01). he absence of Gαi2 in mice is a substrate for ventricular arrhythmia in part accounted for by increased L-type Ca2+ channel activity.