We prepared diet-induced hyperhomocysteinemia (hHcy) in adult male Wistar rats and investigated the effects of hHcy on the adenosinergic and adrenergic responses in vitro and in vivo. The responsiveness of right atria from hHcy rats to the negative chronotropic effects of adenosine (Ado) was found to be significantly greater in hHcy rats than in controls. The pD2 value and maximum effect of Ado were significantly increased in 12-week hHcy right atria when compared with those from age-matched controls. The vasodilatory effect of Ado on rat thoracic aorta was also increased in hHcy rats. In the presence of dipyridamole, an Ado uptake inhibitor, the negative chronotropic and vasodilatory effects of Ado were significantly potentiated in the hHcy rats much more than in the control rats. In anesthetized rats, Ado and dipyridamole, given as a rapid bolus into the femoral artery, led to reduction in mean blood pressure and heart rate. This effect was significantly pronounced in hHcy rats when compared with control animals. Otherwise, hHcy atria were found to have increased responsiveness to the positive chronotropic response to isoproterenol, an β-adrenoceptor agonist. However, there were no significant differences between two groups in the vasoconstrictor effects to phenylephrine, an α-adrenoceptor agonist. On the basis of these results, we concluded that hHcy rats were significantly more sensitive to the negative chronotropic and vasorelaxant effects of Ado, possibly because of accelerated cellular Ado uptake and/or a change in Ado receptor-G protein system. This change may be related with the increased responsiveness to β-adrenergic agonists in hHcy rats, and might contribute to the harmful cardiac effects of hHcy.