We tested whether adenosine mediates nitric oxide (NO)-dependent and NO-independent dilation in coronary and aortic smooth muscle and whether age selectively impairs NO-dependent adenosine relaxation. Responses to adenosine and the relatively nonselective analog 5'-N-ethylcarboxamidoadenosine (NECA) were studied in coronary vessels and aortas from immature (1-2 mo), mature (3-4 mo), and moderately aged (12-18 mo) Wistar and Sprague-Dawley rats. Adenosine and NECA induced biphasic concentration-dependent coronary vasodilation, with data supporting high-sensitivity (pEC50 = 5.2-5.8) and low-sensitivity (pEC50 = 2.3-2.4) adenosine sites. Although sensitivity to adenosine and NECA was unaltered by age, response magnitude declined significantly. Treatment with 50 华 G-nitro-L-arginine methyl ester (L-NAME) markedly inhibited the high-sensitivity site, although response magnitude still declined with age. Aortic sensitivity to adenosine declined with age (pEC50 = 4.7 ᠰ.2, 3.5 ᠰ.2, and 2.9 ᠰ.1 in immature, mature, and moderately aged aortas, respectively), and the adenosine receptor transduction maximum also decreased (16.1 ᠰ.8, 12.9 ᠰ.7, and 9.6 ᠰ.7 mN/mm2 in immature, mature, and moderately aged aortas, respectively). L-NAME decreased aortic sensitivity to adenosine in immature and mature tissues but was ineffective in the moderately aged aorta. Data collectively indicate that 1) adenosine mediates NO-dependent and NO-independent coronary and aortic relaxation, 2) maturation and aging reduce NO-independent and NO-dependent adenosine responses, and3) the age-related decline in aortic response also involves a reduction in the adenosine receptor transduction maximum.