The dependence of heart rate on sympathetic nerve development and on ontogeny of cardiac responses to sympathetic neurotransmitters was evaluated in neonatal rats by use of the ganglionic blocking agent, chlorisondamine, and the beta-adrenoceptor agonist, isoprenaline. 2. Basal heart rates of anaesthetized 4, 7 and 11 day old rats were the same as in adult rats, but the heart rate in 16 day old rats (an age at which sympathetic tone appears to be elevated) was significantly higher. 3. Chlorisondamine failed to lower heart rate significantly in 4 or 7 day old rats, but lowered the rate by 50 to 80 beats per min in the older animals and eliminated the difference in rate between 16 day old and the other ages; atropine had little or no effect on heart rate. 4. Heart rate responses to isoprenaline were lower in 4, 7 and 11 day old rats than they were in adults but were indistinguishable from them by 16 days of age, indicating that the heart is less sensitive to beta-adrenoceptor stimulation early in neonatal life. 5. The ontogeny of sympathetic control of heart rate in the rat depends upon maturational changes in both presynaptic and postsynaptic elements.
