The effects of intravenous nitroglycerin (Lenitral) on the calibre of coronary arteries and their lesions was studied in 100 consecutive patients undergoing coronary angiography. The coronary arteries were divided into segments (3 for the right coronary, 2 for the left anterior descending, and 2 for the circumflex artery). 510 segments were assessed (279 normal, and 231 pathological). The diameter of each was measured through the mid point before and after injection of a bolus of 0.3 mg nitroglycerin. The calibre of the coronary segments increased in 325 cases (63.7%). In 49 cases, the increase was greater than 50%. It remained the same in 151 cases (29,6%) and decreased in 84 cases (6,7%). The average increase in calibre was 16% (p less than 0.01). This had little relation to the artery under study, the location of the segment or whether the segment was pathological or normal. However, two findings were of interest: - the percentage of segments which increased their calibre was significantly higher in the normal segments (191 out of 279 cases, 68%) than in the pathological segments (134 out of 231 cases, 58%) (p less than 0.01). - Coronary lesions may change in appearance: images of stenosis or obstruction completely regressed in 4 cases, the degree of stenosis seemed to increase in 11 cases because of dilatation of the coronary vessel proximal and distal to the narrowing, and distal vessels appeared much more dilated in 5 cases. The factors which seem to affect the vasomotor response are multiple, and include the state of the arterial wall, the presence of vasodilatation or vasoconstriction prior to injection and the mode of administration of the nitroglycerin. The test should be performed by intravenous or intracoronary injection at the end of coronary angiography so as not to mask spastic phenomena. Some errors in the interpretation of the degree of stenosis or the quality of the distal post-stenotic coronary bed could thus be avoided.