Background: Thymus species are widely used herbal medicinal plants for various ailments throughout the developing world Thymus schimperi. T. schimperi is one of the species that is used as spices and traditional medicine for various ailments in Ethiopia.

Objective: This study was designed to assess the acute and subchronic toxic effects of the aqueous leaves extract of T. schimperi on blood parameters, histopathology and biochemicals of liver and kidney of rats.

Methods: The aqueous leaves extract of T. schimperi was tested for toxicity study. Wistar rats were randomly divided into control and treatment groups. The doses for acute toxicity study were single doses of 300, 2000, 5000 and 10,000 mg/kg body weight of animal. Whereas, subchronic toxicity study daily administration of doses of 200 and 600 mg/kg of the aqueous extract were used for 90 consecutive days. Biomarkers and hematological parameters, microscopic examination of liver and kidney tissue and body weight of rats were evaluated following the test period besides recording the signs of toxicity.

Results: Acute toxicity study did not reveal any signs of toxicity; hence the LD50 was higher than 10,000 mg/kg. There was no significant change (p> 0.05) in general body weight and most of evaluated hematological and biochemical parameters after 90 days of sub-chronic treatment. The kidneys and liver of treatment group appear normal in their texture, shape, size or color compared to the control in gross and histopathological examination. However, the light microscopic examination reveals that there was localized mononuclear lymphocytic infiltration and mild blood congestion within the hepatic portal and central veins in liver at higher dose (600 mg/kg).

Conclusion: The findings revealed that the aqueous leaves extract of T. schimperi relatively nontoxic in rats.