The influence of monensin (10 µmol/L) alone and in the presence of verapamil (3 µmol/L) or ouabain (1 mmol) and prazosin (1 µmol/L) or glibenclamide (10 µmol/L) were studied on the muscle tension of guinea pig aorta. Changes in tissue sodium, potassium and calcium ion contents of the aortic muscle produced by monensin in the presence of prazosin were evaluated. Monensin in normal Tyrode's solution containing prazosin caused an increase in the resting tension followed by a decrease and returned to normal values. Verapamil reduced the muscle tension induced by monensin. After ouabain, monensin induced sustained increase in the resting tension. Glibenclamide partially reversed the relaxant phase of monensin in presence of norepinephrine. The findings of the present study indicate that the contraction induced by monensin in guinea Pig aorta is due to the increased influx of calcium through voltage-dependent calcium Channels as well as through Na+ - Ca2+ exchange mechanisms. Monensin produced relaxation of guinea pig aorta in part through the increase in K+ permeability via opening of K+-ATP channels.