Although the number of β-adrenoceptors is unchanged with age in rat heart, both (β-adrenoceptor and postreceptor activation of adenylyl cyclase decreases with age. Pharmacologic data suggest that it is the amount of adenylyl cyclase enzyme units that limit activation of adenylyl cyclase with senescence, but direct quantitation of either G protein or adenylyl cyclase in rat heart with age is lacking. To quantitate the amount of adenylyl cyclase and G proteins with age directly, we assessed for-skolin-stimulated adenylyl cyclase activity, the number of [3H] forskolin binding sites, and stimulatory G protein (Gsα) and inhibitory G protein (Giα) immunoreactivity in the ventricles from 6− and 24-month-old F-344 rats. The amount of Gsα and Giα was unchanged with age in both crude membranes and partially purified membranes from ventricles. In contrast, there was a 32% decrease in the ability of forskolin to stimulate adenylyl cyclase maximally and a 41% decrease in the number of forskolin binding sites with age. Sensitivity for forskolin activation was unchanged with age, but there was a slight increase in affinity for [3H]forskolin binding. The decrease in the amount of adenylyl cyclase with age correlates with the diminished capacity to activate adenylyl cyclase with age and may account for the reduced p-adrenergic signal transduction observed in senescent rat heart.