Previous in vitro studies have demonstrated that coronary artery adrenergic nerves are a principal site of accumulation of 5-hydroxytryptamine released from aggregating platelets. The purpose of this study was to determine whether 5-hydroxytryptamine is accumulated by adrenergic nerves at sites of endothelial damage and platelet aggregation in vivo. Coronary artery 5-hydroxytryptamine content and response to in vitro adrenergic nerve stimulation were studied in dogs 24 hours following balloon catheter-induced intimal injury. 5-Hydroxytryptamine content was significantly increased in the catheter-damaged arteries, and there was a coincident decrease in the content of norepinephrine. The relaxation caused by acetylcholine was abolished in the catheter-injured arteries, indicating loss of this endothelial cell-mediated function. The normal β-adrenergic relaxation caused by nerve stimulation was inhibited, and in some cases, contractions resulted; these effects were prevented by serotonergic receptor antagonists. The sensitivity to exogenously added norepinephrine was unchanged, indicating that the changes in the response to nerve stimulation were not due to an altered smooth muscle response to the native neurotransmitter. These observations indicate that following intimal damage, which produces platelet aggregation on the luminal surface of the blood vessel, 5-hydroxytryptamine can assume a transmitter role in coronary artery adrenergic nerves and thereby cause their dysfunction.