Features of infarction can be divided into two types--the spasmodic and the mechanical. The former (pre-infarct angina and emotional factors in infarction) seem readily explainable by spasm, and are similar to the findings in angina which prompted Heberden to consider angina as spasmodic. The mechanical features of infarction (association with thrombosis and arteriosclerosis, and severe and unremitting chest pain) seem to be the antithesis of spasm and probably account for the reluctance to consider spasm seriously in infarction. The injury-vasospasm hypothesis of acute myocardial infarction explains both spasmodic and mechanical features. Spasm represents a dominance of vasoconstricting over vasodilating forces. Coronary sclerosis can result in both ischaemia (vasodilating) and ischaemic injury-spasm (vasoconstricting). The fight-flight component of the autonomic nervous system is considered to be vasodilating, and the conservation-withdrawal portion to be vasoconstricting. Once spasm occurs, a new balance of forces obtains which can lead either to vasodilatation and relief of symptoms or to infarction.