The direct effects of sympathetic coronary innervation in conscious dogs were analyzed by comparison of myocardial blood flow in a sympathectomized section of the left ventricle with blood flow in another section of the same ventricle with intact sympathetic innervation. Regional myocardial sympathectomy was accomplished by a protocol of intracoronary application of 6-hydroxydopamine (6-OHDA). Regional adrenergic nerve function was checked by studies on accumulation, retention and stimulus-induced release of labelled norepinephrine together with studies on regional ventricular contractility during stellate ganglion stimulation. These tests demonstrated complete sympathectomy in the 6-OHDA-treated section and an unimpaired adrenergic nerve function in the untreated control section. In these dogs at rest, myocardial blood flow (MBF) was 66±14 ml/min·100 g in the left ventricular control section and 108±16 ml/min·100 g in the 6-OHDA-treated section. Transmural distribution of MBF was similar in these two sections: subendocardial MBF/subepicardial MBF was 1.28±0.17 in the control section and 1.36±0.12 in the 6-OHDA-treated section. Application of 1.0 mg/kg propranolol i.v. did not modify MBF and its distribution in either section. Following 1.0 mg/kg phentolamine i.v., MBF in the control section approached that in the 6-OHDA-treated sympathectomized section. These results demonstrate that a substantial α-adrenergic constrictive coronary tone is operative in the ventricular control section in these conscious dogs at rest, which is of similar magnitude in different layers of the left ventricular wall.