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The guanethidine sympathectomy in 4- and 8-week old white rats increased cardiac sensitivity to acetylcholine. An increase of sensitivity to adrenaline was observed in 8 to 20 weeks of postnatal period. The significance of the changes of extracardiac effects and the cardiac sensitivity to acetylcholine and adrenaline for its chronotropic control in sympathectomized 10-14 week old rats is discussed.

A differentiated rheogram was recorded in anesthetized rats at the age of 1, 3, 5, 7, 11, 14, 21, 30, 42, 49, 70 and 100 days. Their cardiac output was found to increase with advancing age and played a major part in an increase of the minute-volume output, being more important than the heart rate. The minute-volume of the circulation as related to 100 g body weight proved to be a constant parameter during postnatal ontogenesis. The specific resistance of the blood and the hematocrit increased with advancing age, the dependence of the former on the latter having been expressed by a worked out formula.

Catecholamine level in the blood, heart and adrenal tissues, and heart sensitivity to catecholamines were determined in 3-week-old rats after chemical sympathectomy. It was shown that tachycardia in sympathectomy rats was caused by increased heart sensitivity to catecholamines due to the reduction in their concentration.

Using pharmacological drugs, studies have been made on the development of sympathetic and parasympathetic innervation of the heart in ontogenesis of dogs. It is concluded that sympathetic and parasympathetic innervation operates already at the delivery of animals. Originally their effect is observed only as regulation of cardiac pacemakers, influences on the contractile properties of the heart being exerted in later ontogenesis.