The concentrations of norepinephrine (NE), epinephrine (E) and dopamine (DA) were determined in dog hearts, each divided into a total of 18 parts. The concentration of NE was found to be significantly (three times) higher in the atria than in the ventricles while smaller differences existed within various subdivisions. In particular, the left atrial body had a lower concentration than other atrial tissue. The same was true of the left ventricular apex as compared to the base. No significant differences in NE were observed between the average concentrations in epicardial vs. endocardial layers of left ventricle, nor in the S-A or A-V node areas vs. surrounding tissues.The concentration of DA was also higher in atria than in ventricles. However there was no consistent correlation between NE and DA concentrations in the various atrial and ventricular parts. In the ventricles, DA was higher in left than in right ventricle and still higher in the A-V node region. In the atria DA was highest in the S-A node region and least in the atrial body. Both right and left atrial appendages contained high DA concentrations especially in their upper parts. In these parts and in the S-A node region the DA concentration was nearly equal to or slightly higher than that of NE. In most other areas DA was one-fifth to one-tenth the NE concentration.By contrast E was present in very low concentrations amounting on the average to less than 5% the amount of NE in the same tissue. No distinct pattern of E distribution in various parts of the heart could be established.It is concluded that no sample of the heart can be considered as representative of the entire organ, and that the catecholamine content of the entire heart cannot be taken as reflecting the concentrations in various specific areas.