The purpose of this study is to examine quantitatively the branching characteristics of the coronary arteries. Branching angles and vessel diameters were measured in a total of 175 arterial bifurcations in the coronary beds of rats, and the results are compared with those of 350 bifurcations in other parts of the cardiovascular system of the same species. Significant differences are found in the values of branch diameters and branching angles, both being found generally lower in the coronary bed than in other parts of the system. On statistical grounds these differences are found to have very high significance levels, with P values less than 0.02 in the case of branching angles and much less than 0.001 in the case of branch diameters. On physiological grounds, the differences are such as to place the coronary arteries further away from the "theoretical optimum" than are vessels in other parts of the cardiovascular system. The theoretical optimum represents branching angles and branch diameters which make arterial bifurcations more efficient physiologically.