Cardiac membranes from 26-, 52- and 104-week-old Wister rats were used to investigate the age-related alterations in the β-adrenergic receptor-adenylyl cyclase system. The densities and affinities of β-adrenoceptors did not change with aging. There were no significant changes in the total amount of stimulator G-protein (G_s), and in G_s activity measured in a reconstitution assay using human platelet membranes. The major isoform of G_sα, however, changed from a 45.000 to 52.000 dalton peptide with aging. The total amount of pertussis toxin substrates (G_i2 and G_o) decreased significantly with aging. This finding was supported by the fact that pertussis toxin-induced potentiation of adenylyl cyclase activity was markedly reduced in the aged group. The activity of catalytic protein assessed by forskolin-stimulated adenylyl cyclase activity was de-creased at 104 weeks. On the other hand, GTP analogue-stimulated adenylyl cyclase activity was significantly potentiated in the same group. These results suggest that the decreased sensitivity to catecholamines observed in aged hearts is mainly due to a dysfunction of catalytic protein, and that decreased G_i activity partially compensates for this catalytic dysfunction.