—L-type Ca²⁺ channels contribute importantly to the normal excitation-contraction coupling of physiological hearts, and to the functional derangement seen in heart failure. Although Ca²⁺ channel auxiliary β_1–4 subunits are among the strongest modulators of channel properties, little is known about their role in regulating channel behavior in actual heart cells. Current understanding draws almost exclusively from heterologous expression of recombinant subunits in model systems, which may differ from cardiocytes. To study β-subunit effects in the cardiac setting, we here used an adenoviral-component gene-delivery strategy to express recombinant β subunits in young adult ventricular myocytes cultured from 4- to 6-week-old rats. The main results were the following. (1) A component system of replication-deficient adenovirus, poly-l-lysine, and expression plasmids encoding β subunits could be optimized to transfect young adult myocytes with 1% to 10% efficiency. (2) A reporter gene strategy based on green fluorescent protein (GFP) could be used to identify successfully transfected cells. Because fusion of GFP to β subunits altered intrinsic β-subunit properties, we favored the use of a bicistronic expression plasmid encoding both GFP and a β subunit. (3) Despite the heteromultimeric composition of L-type channels (composed of α_1C, β, and α₂δ), expression of recombinant β subunits alone enhanced Ca²⁺ channel current density up to 3- to 4-fold, which argues that β subunits are “rate limiting” for expression of current in heart. (4) Overexpression of the putative “cardiac” β_2a subunit more than halved the rate of voltage-dependent inactivation at +10 mV. This result demonstrates that β subunits can tune inactivation in the myocardium and suggests that other β subunits may be functionally dominant in the heart. Overall, this study points to the possible therapeutic potential of β subunits to ameliorate contractile dysfunction and excitability in heart failure.