Effects of different concentrations of monensin on contraction amplitude (CA) which is the contraction force (CF), initial contraction velocity (ICV), average contraction velocity (ACV) and initial relaxation velocity (IRV) of guinea-pig papillary muscle were studied by using male guinea-pigs. Hearts were removed and papillary muscles were isolated from the right ventricles. The muscles were then attached to a capacitance transducer, electrically connected to a Beckman recorder and the isometric contractions of guinea-pig papillary muscles were recorded. The five hours of measurement in this experiment were divided into five periods (T0 was equilibration, T1, T2, T3 and T4 were respectively one, two, three and four hours after drug administration). Two mu mol/l monensin did not change any parameters at any time period. Although 12 mu mol/l monensin did not change the CA, ACV or IRV at any time period, it increased the ICV at T2. Seven mu mol/l monensin, on the other hand, increased the CA and IRV at T2, T3 and T4 and the ICV and ACV at T1, T2, T3 and T4. It is concluded that the effect of monensin in guinea-pig papillary muscles is dose dependent, It is also concluded that although the therapeutic range of monensin is very narrow, it should be considered as an alternative drug for the treatment of congestive heart failure (CHF). However, more studies are needed to domenstrate the effectiveness of monensin on the tratment of congestive heart failure.