The aim of this study was to determine the Na+/K(+)ATPase activity in the erythrocytes of sheep naturally infected with Babesia ovis before and after treatment. Seven diseased sheep and seven control animals were used for the study. Babesia infection was confirmed with Giemsa's staining of blood smears. Na+/K(+)ATPase activity in erythrocyte was determined colorimetrically by the release of inorganic phosphate from ATP in the presence and absence of ouabain. A marked decrease of Na+/K(+)ATPase activity (p < 0.05) was obtained in infected sheep (0.81 +/- 0.28 mu mol inorganic phosphate/h.mg protein) compared to control animals (3.63 +/- 0.72 mu mol Pi/h.mg protein). Beside this decline, Na+/K(+)ATPase activity of treated animals (1.29 +/- 0.81 mu mol Pi/h.mg protein) were significantly (p < 0.05) decreased compared to control animals. It can be concluded that decreased erythrocyte Na+/K(+)ATPase activity in babesia ovis may be due to the usage of new permeation pathways (NPPs) for the flux of ions instead of Na+/K+ pump.