Chlordimeform N'-(4-choro-O-tolyl)-N,N-dimethyl-formamidine; CDF), an acaracide-insecticide, relaxed helically cut strips of rabbit thoracic aorta and deadventitiated aorta strips. Relaxations by CDF of aorta strips contracted by 5 x ~O -' M prostaglandin Fza were not altered by the antimuscarinic agent atropine (lo-' M), the betu-adrenergic antagonist propranolol lo-'^), the histamine Hz antagonist cimetidine (2 x 1 0 -4 ~) , the dopamine antagonist haloperidol M) or the Na+-K+-ATPase inhibitor ouabain (5 x 1 0 -6 ~) . CDF further relaxed deadventitiated strips contracted by 1 0 -6 ~ NE or 40 mM potassium and then partially relaxed by lo-' M verapamil. CDF M) potentiated contractions caused by the addition of calcium to potassium depolarized strips in zero calcium media, but higher concentrations reduced calcium-induced contractions. The hwhest CDF concentration (lo-' M) not only prevented tension increase on calcium addition, but also progressively decreased tension to below baseline levels. The addition of 1 0 -2 ~ CDF to aorta strips not contracted by a vasoactive agent decreased resting tension. This relaxation was usually maximal at 1 0 -3 ~ and less at 10-2M. Thus, CDF relaxes vascular smooth muscle, not by antagonism at the usual vascular relaxant receptors, but by interference with calcium utilization.