SummaryThe intra-venous injection of 75 mgr of nicotinic acid determines a fall of the intra-arterial blood pressure, usually starting two minutes after the injection and lasting for about 5 to 10 minutes. In the studied cases, the cardiac output increases from 6 to 150 % according to tho cases. A marked acceleration of the circulation (oxymetric method) was noted. Some patients presented an increase of the oxygen consumption probably due to the hyperpnea. The blood volume, studied by the Gregersen method (intra-venous injection of T 1824) did not show any important changes. The plasmatic volume, after nicotinic acid injection shows an average value of 98 % of its initial level. This difference has no statistical signification.The increase of the cardiac output can be partly explained (at least in some cases) by the increase of the metabolism. It is yet mostly due to the peripheric vaso-dilatation.Such circulatory modifications make the nicotinic acid unsuitable for the treatment of coronary diseases.