Recent reports revealed that the Na⁢-Ca2⁢ exchangers and feet structures of sarcoplasmic reticulum(SR) are located in close vicinity in the specific compartment. Therefore, we investigated the possibility that the Na⁢-Ca2⁢ exchanger may decrease the tension development by transporting the Ca2⁢ out of the cell right after it released from SR, on the basis of this anatomical proximity. We exammined the negative force-frequency relationship of the developed tension in the electrically field stimulated left atria of postnatal developing rat(1, 3 day, 1 week and 4 week old after birth). Cyclopiazonic acid(3x10-5 M) treatment decreased the developed tension further according to postnatal age. Monensin(3x10-6 M) treatment did not increase the maximal tension in 4 week-old rat, preserving negative staircase, while the negative staircase in the younger rat were flattened. Ca2⁢ depletion in the buffer elicited more suppression of the maximal tension according to the frequency in all groups except the 4 week-old group. The % decrease of the maximal developed tension of 4 week-old group at 1 Hz to that of 0.1 Hz after Na⁢ and Ca2⁢ depletion was only a half of those of the yonger groups. Taken together, it is concluded that the Na⁢-Ca2⁢ exchange transports more Ca2⁢ released from SR out of the cell in proportion to the frequency, and this is responsible for the negative staircase effect of the rat heart.