1. Excised rings of the coronary artery of the ox properly weighted react to epinephrin by dilatation.
2. This reaction of the ox coronary is not an active dilatation, but is in the nature of a sudden lowering from a high to a relatively low degree of tonus. The tonus is not, however, entirely abolished by epinephrin.
3. The degree of relaxation produced by epinephrin is not constant. It depends on several factors other than the concentration of epinephrin used.
4. It is possible to increase the sensitiveness of the coronary artery to epinephrin.
5. There is no evidence of any primary constrictor effect on the ox coronary from epinephrin in most dilute solutions, nor can the existence in this artery of any constrictor mechanism of sympathetic origin be determined by the excised ring method.