Potent antihyperlipaemic activity has been observed in a series of novel N-cyanovinylformamidines when tested in hyperlipaemic rats. Two of the compounds (11 and 15) were found more potent than gemfibrozil at 50 mg/kg/d dose level in reducing serum cholesterol and triglyceride levels and also in elevating serum HDL level. A good three-dimensional structural similarity has been observed between these two compounds and clofibrate and gemfibrozil, respectively. Acute toxicity studies carried out in mice indicated compound 11 to be safe even at a dose level of 4.5 g/kg. The title compounds were synthesized by the reaction of alpha-cyanoketene S,N-acetals with formamidineacetate under controlled reaction conditions.