Background: Hypertension is associated with sympathetic hyperactivity and tachycardia. We tested the hypothesis that b-adrenergic hyper-responsiveness in the sino-atrial node (SAN) of the spontaneously hypertensive rat (SHR) occurs at the level of the L-type calcium current (I_CaL ), due to altered nitric oxide (NO) cyclic nucleotide-dependent signalling.Methods and Results: Heart rate responsiveness to norepinephrine (NE), together with production of cAMP and cGMP was assessed in isolated atrial preparations from age-matched SHRs and normotensive Wistar-Kyoto rats (WKY). Right atrial / SAN pacemaking tissue was injected with adenoviral vectors encoding enhanced green fluorescent protein or nNOS (Ad.nNOS). I_CaL was measured in cells isolated from the SAN of transfected animals. SHR atria were hyper-responsive to NE at all concentrations tested, with elevated production of cAMP, but with no difference in cGMP. This was accompanied by increased basal and NE-stimulated I_CaL . Injection of Ad.nNOS increased right atrial nNOS protein expression, normalised chronotropic responsiveness of SHR atria to NE, and attenuated NE-stimulated I_CaL.Conclusions: Decreased I_CaL following cardiac nNOS gene transfer contributes to the normalisation of b-adrenergic hyper-responsiveness of rate in hypertensive rats.