Beta-pyridylcarbinol (Ronicol) when injected (1,2-1,4 mg/kg i.v.) into four patients with arteriosclerosis obliterans caused a release of an unstable disaggregatory substance into arterial blood of those patients. This release was not observed in other two patients who had been pretreated with aspirin (1,8 g p.o.daily). It is postulated that some of pharmacological properties of nicotinic acid derivatives are mediated through the release of endogenous PGI2-like substance.