Thymus (Labiatae) is one of the herbs used in the traditional medicinernpractice in Ethiopia. The claimed actions of thymus are carminative, antimicrobial,rnanti-spasmodic, relaxing expectorant, astringent, anthelmentic andrnanti-oxidative. But some people claim and have concern, that thymus causesrnproblems to liver in herbivorous animals that graze on it and human beingsrnwho ingest it for different purposes.rnThe aim of the present study was to evaluate the claimed spasmolytic effect and safety ofrnThymus serrulatus on liver of rats.rnExperimental study was done in laboratory animals. Charcoal meal test was used tornevaluate its spasmolytic effect on mice. Acute toxicity was evaluated by determining LD50rnof the plant extracts on mice. Subchronic toxicity was done on rats. Rats were divided intorneight groups of five animals each. Four groups were female and the other four were male.rnThree different doses, 0.2 gm/kg, 1 gm/kg and 2 gm/kg of the aqueous extract were givenrnto the six treatment groups via oral gavage for 90 days. During the study period thernanimals were observed for any sign of liver toxicity and their body weight was measuredrnweekly. At the end of the study period clinical chemistry parameters, gross morphologyrnand histopathology of the liver were evaluated.rnThe results showed the presence of spasmolytic effect, which is statistically significantrndifference (P0.05). Thernoral LD50 for the extract was seen to be higher than 5 gm/kg for both aqueous andrnmethanolic extracts, which is indicative of the plants safety in case of acute high dosernexposure. Cage side observation in subchronic toxicity study revealed no sign of liverrntoxicity. No statistically significant difference (P>0.05) was seen in the clinical chemistryrnresults between the different groups. However, the histologic evaluation revealedrncholestatic change of the liver in most of the animals in the treatment group. This may bernexplained by the presence of phytosterol compounds, in the extract given, which arernknown to cause cholestasis. Cholestasis was not accompanied by necrosis/degeneration,rnwhich could be due to the anti-oxidant and hence protective effect of the active ingredientrnthymol. In conclusion T. serrulatus has spasmolytic activity and is safe for oral use.rnKey words: T.serrulatus, toxicity, laboratory animals, extracts.