A mechanism for the genesis of early relaxation of the left ventricle was assessed. For 18 patients with coronary artery disease, biplane left coronary cineangiography was performed and the coordinates (x, y, z) of the points of the artery were measured by frame to frame basis throughout one cardiac cycle. The spatial distance (segment length) between any two points was calculated on each frame as (Formula: See text) to investigate shortening and relaxation characteristics of the ischemic and nonischemic segments. Coronary angiography was repeated during right atrial pacing to aggravate ischemia, and following nitroglycerin administration to ameliorate ischemia. From data of six normal subjects, 43 segments were calculated. All showed shortening during systole and lengthening in diastole. Right atrial pacing caused early relaxation in only two of the 43. Two hundred and fifteen segments were calculated for 18 patients with coronary artery disease. All but eight segments showed normal relaxation on control angiography. Following pacing, 115 segments showed early relaxation, and in 107 of the 115, normal relaxation were restored after nitroglycerin. Among the 107 segments, 44 were ischemic and 63 were normal. For 34 segments with early relaxation by pacing, pacing was repeated after administering nitroglycerin, and 15 did not reproduce early relaxation. The present study showed that early relaxation was observed in patients with coronary artery disease, which was effectively ameliorated by nitroglycerin, suggesting this phenomenon is closely related to the development of myocardial ischemia. The decreased active tension, reduced duration of tension, development and delay in electrical depolarization in the ischemic segment can be a reasonable inducement to early relaxation.(ABSTRACT TRUNCATED AT 250 WORDS)