1. Studies were performed in porcine left circumflex coronary arteries to determine the time course of changes in their responses to smooth muscle and endothelial cell agonists following death. 2. The pigs were kept at room temperature for 3, 6, 12 and 24 hr after death before removing the arteries, and the responses compared with fresh arteries. Rings of the arteries were mounted in organ chambers filled with modified Krebs-Ringer solution for isometric tension recording. 3. Fresh rings with endothelium relaxed with stretching, whereas 3- and 6-hr rings developed an increase in tension and often rhythmic activity. In 3-hr rings, there were diminished contractile responses to KCl and serotonin. The contractile responses to prostaglandin F2 alpha were depressed at 3 and 6 hr. 4. At 6 hr, rings from six of 14 pigs failed to contract and the responses of the other eight to histamine and serotonin but not to KCl or acetylcholine, were depressed compared to fresh rings. 5. 6 hr after death endothelium-mediated relaxations to bradykinin and calcium ionophore A-23187 were depressed while endothelium-mediated as well as direct smooth muscle relaxation by adenosine diphosphate did not change. 6. These studies indicate that caution must be observed in interpreting the findings in autopsy specimens examined as early as 3 hr after death.