The aqueous fraction of an aqueous-alcoholic extract of Hibiscus sabdariffa L. calyces was given to Wistar albino rats (150-200g) orally as drugs to study the toxicity of the extract. The rats, which were fed with commercial diet and water ad libitum, were divided into six groups of four rats each. Treatments 1 through 6 received 0, 1, 3, 5, 10 and 15 doses of 250 mg/kg each, respectively; the control group received physiological saline (0.9% NaCl). Results of the studies showed that the levels of serum aspartate aminotransferase (AST) and alanine amino transferase (ALT) were significantly (P<0.05) increased in all the treatments compared with the control group. However, the serum levels of alkaline phosphatase, and lactate dehydrogenase were not significantly (P>0.05) affected. Only the group with 15 doses had their serum level of albumin significantly (P<0.05) increased. However, the results of histopathological studies showed that both the livers and hearts gave no pathological features for all the treatments. The results showed that prolong usage of this extract at 15-dose level could cause liver injury while the effect was mild at small dose levels (1-10). Though the average consumption of 150-180 mg/kg per day appears safe, the extract should be taken with caution bearing in mind that higher doses could affect the liver.