Effects of ouabain (10^-12M to 10^-5M) on the simultaneously recorded transmembrane action potential and contraction; and (Na⁺-K⁺)-activated membrane ATPase of human papillary muscles were investigated. Ouabain in the concentration of 10^-12M did not affect the transmembrane potential, contraction and (Na⁺-K⁺)-ATPase activity. Ouabain in the concentrations of 10^-11 to 10^-7M produced a concentration dependent inhibition of (Na⁺-K⁺)-APTase associated with shortening of action potential duration and increase in contraction. Ouabain (10^-7M) produced a time dependent inhibition of the (Na⁺-K⁺)-ATPase associated with the shortening of action potential duration and increase in contraction. Ouabain in the concentration of 10^-5M produced an initial marked increase in contraction which was shortly followed by a decrease in contraction and an increase in tone. The muscles developed contracture within 15 to 20min. This concentration of ouabain inhibited the (Na⁺-K⁺)-ATPase completely. These results suggest that there is a relation between the inhibition of the (Na⁺-K⁺)-ATPase and shortening of action potential duration associated with an increase in contraction. Human papillary muscles seem to be very sensitive to the effect of ouabain.