Transgenic overexpression of Gα_q causes cardiac hypertrophy and depressed contractile responses to β-adrenergic receptor agonists. The electrophysiological basis of the altered myocardial function was examined in left ventricular myocytes isolated from transgenic (Gα_q) mice. Action potential duration was significantly prolonged in Gα_q compared with nontransgenic (NTG) myocytes. The densities of inward rectifier K⁺ currents, transient outward K⁺ currents (I _to), and Na⁺/Ca²⁺ exchange currents were reduced in Gα_q myocytes. Consistent with functional measurements, Na⁺/Ca²⁺ exchanger gene expression was reduced in Gα_q hearts. Kinetics or sensitivity ofI _to to 4-aminopyridine was unchanged, but 4-aminopyridine prolonged the action potential more in Gα_q myocytes. Isoproterenol increased L-type Ca²⁺ currents (I _Ca) in both groups, with a similar EC₅₀, but the maximal response in Gα_q myocytes was ∼24% of that in NTG myocytes. In NTG myocytes, the maximal increase of I _Ca with isoproterenol or forskolin was similar. In Gα_q myocytes, forskolin was more effective and enhanced I _Ca up to ∼55% of that in NTG myocytes. These results indicate that the changes in ionic currents and multiple defects in the β-adrenergic receptor/Ca²⁺ channel signaling pathway contribute to altered ventricular function in this model of cardiac hypertrophy.