During the last decade expensive studies on catecholamines have evolved new knowledge in the
physiology and biochemistry of adrenergic mechanism. Cardiac muscle, receiving adrenergic fibres
from the stellate, cervical and thoracic ganglia, has been repeatedly shown to have a specific capacity
to uptake and to store catecholamines. The catecholamine stores in cardiac muscle have also been
shown to be important sites for the action of numerous drugs. Under normal condition, a certain
level of catecholamines is maintained in the stores and serves as the basis for studying the changes
in the catecholamine content of the heart. Because myocardial catecholamines play such important
role in the patho-physiology of the heart, it would be interesting to compare the normal level of
myocardial catecholamines among various species of animals. An occasional study has dealt with
myocardial catecholamines of several species and ages of animals but these have been insufficiently
comprehensive to afford a basis for an understanding of the importance of these amines as related to
species and ages. The present investigation was undertaken to determine whether or not there is any
significance of myocardial catecholamines in the c::mrse of the evolution and development of animals.
Seasonal changes, sex difference and regional and subcellular distribution of myocardial catecholamines
were also examined. The concentration of cardiac catecholamines was determined by the
spectrophotofluorometric procedure described by Shore and Oiin. rat, cow and pig. The ratio of norepinephrine to epinephrine also was greater as the animals progress phylogenetically. 2. Examination of the regional distribution of cardiac catecholamines in warm-blooded animals showed that the content of the auricle was generally higher than that of the septum and considerably than that of the ventricle, but the differences of contents among these regions were not so marked. 3. In the embryonic chick, cardiac catecholamines were firstly detected on the 4th day of incubation, the time before the cardiac innervation of sympathetic nerves. The concentrations of these catecholamines increased but not markedly on the 6th day of incubation, soon after the innervation of sympathetic nerves to the heart. The level of the cardiac catecholamines fluctuated throughout the remainder of embryonic development. 4. In newborn rat hearts, a considerable amount of catecholamines was present. With the development of the rats, the concentrations of myocardial catecholamines increased. The ratio of epinephrine and norepinephrine fluctuated within the range of 40 to 60 pervent. However, as development progressed, the percentage of norepinephrine continued to rise, attaining the adult value of
