Responses to vasoactive agents were compared in helical strips of coronary arteries isolated from beagles of 30 days, 3 months, 2 years and 12 years old. Serotonin contracted the arterial strips dose-dependently, the contraction being greater in the arteries of proximal portion than in the distal arteries. The contraction increases with age from 3 months to 12 years, although EC50 values did not differ. Angiotensin II contracted distal coronary arteries to a greater extent than the proximal ones. Age did not alter the peptide-induced contraction. In prostaglandin F2 alpha-contracted coronary arteries, acetylcholine-induced relaxations, dependent on endothelium, were less in the arteries from senescent beagles than in those from adult beagles (2 years old). Histamine relaxed the infant beagle arteries to a lesser extent than the adult and senescent beagle arteries. Histamine-induced relaxations were attenuated selectively by cimetidine. Relaxations caused by adenosine and prostaglandin I2 did not differ in coronary arteries from beagles of different ages. It may be concluded that greater responsiveness to serotonin of senescent beagle coronary arteries is due preferentially to increased function of serotonergic receptors rather than impaired function of the arterial endothelium responsible for the release of relaxing factor(s), although some impairment of the function is supposed, on the basis of interferences with acetylcholine-induced relaxation in the aged beagle arteries. Histaminergic H2 receptor function appears to develop in beagle coronary arteries until 3 months of age.