Aging impairs sympathetic and parasympathetic cardiac control. Although the reduced sympathetic responses are known to depend on an age-related cardiac beta-adrenoceptor dysfunction, the hypothesis of a parallel cardiac muscarinic receptor dysfunction underlying the reduced parasympathetic responses has never been tested. We therefore measured the bradycardic responses to graded electrical stimulations of the right efferent vagus and to graded bolus intravenous injections of acetylcholine in anesthetized, vagotomized rats of young (16 wk) and old (103 wk) age. Unexpectedly, the bradycardia was markedly larger (greater than 2-fold) in old than in young rats with both the electrical and the pharmacological stimulus. This indicates that at variance with its effects on beta-adrenergic receptor responsiveness, aging not only fails to impair but actually enhances cardiac muscarinic receptor responsiveness. It also suggests the more general conclusion that aging has complex and diversified effects rather than simply and uniformly depressing biological functions.