Mitragyna ciliata (MYTA) (Rubiaceae) inhibits plasmodia activity. MYTA induces a cardiotonicity of the digitalic type on rat's isolated heart. In this work we studied the effect of MYTA on microsomal Na(+)/K(+) dependant ATPase (Na(+), K(+) ATPase) extracted from the heart of a rabbit since digitalics inhibit Na(+), K(+) ATPase. Our results revealed that the Na(+)/K(+) ATPase has an optimum pH of 7.4 and temperature of 37 degrees C respectively. There is a linear relationship between the organic phosphate formed and the incubation time over 25 mins incubation period. The ATP hydrolysis rate in the presence of MYTA was 0.775 microM/min. LINEWEAVER and BURK plots showed that MYTA did not alter K(M) (1.31 mM) but decreased V(MAX). This study shows that MYTA exerts a non-competitive inhibition on the microsomal Na(+)/K(+) ATPase extracted from rabbit heart with a Ci(50) of 48 microg/ml. We conclude that the mechanism of action of MYTA is linked to the inhibition of the Na(+)/K(+) ATPase like cardiotonics of the digitalic type.