To determine the modulatory action of adenosinereceptor stimulation on [Ca2+]i responses to β-adrenoceptor stimulation in the heart of the spontaneously hypertensive rat (SHR), the electrically induced [Ca2+]i transient in response to isoproterenol (ISO) in single ventricular myocytes pretreated with adenosine agonists in SHRs and its normotensive control Wistar-Kyoto (WKY) rats was measured with a spectrofluorometric method by using fura-2/AM as the calcium indicator. In both types of rat, ISO at 0.001-1 μM augmented the electrically induced [Ca2+]i transient, and the effect was blocked by a β-adrenoceptor blocker, propranolol. In SHRs that did not exhibit cardiac hypertrophy, the resting level of [Ca2+]i and the amplitude of the electrically induced [Ca2+]i transient were the same as those in WKY rats, whereas the augmentation of the electrically induced [Ca2+]i transient in response to ISO was significantly lower than that in WKY rats. In WKY rats, the effects of ISO on the electrically induced [Ca2+]i transient were inhibited by the adenosine A1-receptor agonist, R(−)-N6-(2-phenylisopropyl)adenosine (R-PIA) at 0.01-10 μM. In contrast, the effects of ISO were further enhanced by the adenosine A2-receptor agonist, N6-[2-(3,5-dimethoxyphenyl)-2-(2-methylphenyl)-ethyl)]adenosine (DPMA) at 1-10 μM. In SHRs, the inhibitory effect of R-PIA was significantly reduced, whereas the excitatory effect of DPMA was absent. The effects of both adenosine-receptor agonists in both types of rat were abolished by the respective adenosine-receptor antagonists, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) and 3,7-dimethyl-1-propargylxanthine (DMPX). The results indicate that the modulatory actions of adenosine-receptor stimulation on [Ca2+]i response to β-adrenoceptor stimulation in the hearts of SHRs are reduced, which is independent of cardiac hypertrophy.