The endothelium releases a powerful vasodilating and anti-aggregant substance, the EDRF, both under basal conditions and after stimulation by a large number of hormones and endogenic factors. Endothelial lesions or dysfunction may play important roles in causing coronary spasm although other changes related to atherosclerosis must also be taken into consideration. One possible stimulus is aggregating platelets as they are an important source of vasoconstricting substances. The endothelium also secretes other vasoactive substances, EDHF and/or EDCF, the physiopathological role of which has not yet been established.