The effects of the essential oil from the leaves of Mentha longifolia L. subsp. capensis on some biochemical parameters of Wistar rats were studied. The oil at 125, 250, 375, and $500;{mu}L/kg$ of body weight reduced (P < .05) the red blood cells and lymphocytes with no definite pattern on the white blood cells and mean cell volume. The doses significantly increased the neutrophils, monocytes, large unstained cells, liver-body weight ratio, and serum concentrations of cholesterol, triglyceride, high-density lipoprotein-cholesterol, and inorganic phosphate but had no effect on the heart body weight ratio and serum low-density lipoprotein-cholesterol, $Na^+$, $Ca^{2+}$, $Cl^-$, $K^+$, creatinine, and uric acid. The oil at $500;{mu}L/kg$ of body weight also increased the kidney-body weight ratio. In contrast, the oil reduced the serum urea and atherogenic index. The total and conjugated bilirubin, together with the total protein and albumin, in the serum increased only with oil at $125;{mu}L/kg$ of body weight. The serum alkaline phosphatase activity also increased with no significant change in those of $gamma$-glutamyl transferase and alanine and aspartate aminotransferase. The results indicate dose- and parameter-specific effect of the essential oil. Although the essential oil from M. longifolia leaves may not predispose to atherosclerosis, it may increase the functional activity of the rat liver at the lowest dose investigated. Therefore, the essential oil from M. longifolia may not be completely "safe" at the doses investigated.