Introduction:We evaluated the sub-chronic toxicity of the aqueous herbal extract prepared from Cassytha filiformis and administered daily for 28 days at dose levels (250, 500, and 1000 mg/kg bw) in male wistar albino rats. The LD 50 of the aqueous extract was determined.Methods: The effects on body weights, organ weights, and certain haematological and plasma biochemical parameters were measured as indices of organ toxicity.Results: The aqueous extract did not affect plasma glutamate oxaloacetate transaminase (GOT) and glutamate pyruvate transaminase (GPT); however, a significant reduction in alkaline phosphatase (ALP) level occurred in all the treated groups. It also did not affect the electrolytes (Naϩ, ClϪ and Kϩ), total and direct bilirubin, creatinine, and glucose level. The aqueous extract elicited hypercholesterolaemic effects, but it did not affect the Hb, WBC, RBC, PVC, platelets, MCH, MCHC, MCV levels and differential counts (lympocytes, neutrophils, monocytes, eosinophils and basophils). It also reduced the body weight gain and absolute weight of the kidneys. The relative weights of the heart and lungs in some animal groups were equally reduced. The acute toxicological evaluation of the plant extract revealed an oral LD 50 value greater than 500 mg/kg bw.Conclusion: This study suggests that aqueous extract of C. filiformis administered at normal therapeutic doses is not likely to produce severe toxic effects on some organs or haematological and biochemical indices in rats.