With increasing age, cardiac β-adrenoceptor function decreases. To study possible mechanisms underlying this process, we assessed in right atrial appendages from 52 patients of different ages (group A, 50 years, mean age 66.1 ± 1.5 years, n = 23) without apparent heart failure who were undergoing open heart surgery β-adrenoceptor number and subtype distribution (by (- )-[125I]-iodocyanopindolol [ICYP] binding), adenylyl cyclase activity, and Gs- and Gi-protein α-subunits (by quantitative Western blotting). β-Adrenoceptor number in the three groups was not significantly different; in contrast, basal, 10 μM GTP-, 100 μM isoprenaline (ISO), 10 mM NaF-, 100 μM forskolin-, and 10 mM Mn2 + -stimulated adenylyl cyclase activity was significantly higher in group A than in group B and was further decreased in group C. Similarly, 100 μM terbutaline-, 100 μM histamine-, and 100 μM 5-HT-stimulated adenylyl cyclase activity significantly decreased from group A to group C. Moreover, all these adenylyl cyclase parameters were significantly negatively correlated with the age of the patients. Although Gsα was not altered, Giα in group C was significantly higher than in group A; moreover, there was a weak but significant positive correlation between Giα and the age of the patients. We conclude that an impairment of the activity of the catalytic unit of adenylyl cyclase is involved in the decrease in cardiac β-adrenoceptor function with age; an increase in Giα might contribute further to the reduced p-adrenoceptor function.