Abstract: Paulinia cupana (Guarana) was evaluated for cytological and biochemical toxicity in male mice. Mice were orally treated with 133, 265 and 530 mg/kg/day of the aqueous suspension of Guarana for 7 consecutive days. The following parameters were evaluated: (I) studies on micronucleus test, (ii) analysis of spermatozoa abnormalities, (iii) analysis of meiotic chromosomes, (iv) quantification of proteins, Ribose Nucleic Acid (RNA) and Deoxy Ribose Nucleic Acid (DNA) in hepatic and testicular cells and (v) estimation of malondialdehyde (MDA) and nonprotein sulfhydryl (NP-SH) in hepatic and testicular cells. The treatment caused significant changes in the frequency of micronuclei in the femoral cells and induced testicular chromosomal aberrations. However, there was no effect on the morphology of spermatozoa. The study on biochemical parameters showed increase of MDA and depletion of NP-SH, RNA and DNA in both hepatic and testicular cells. Present data elucidated the role of free radical species in cytological and biochemical changes in both somatic and germ cells. The exact mechanism of the genesis of lipid peroxides is not known, however, this might be related to the influence of the tannin contents of Guarana. It is suggested that, in view of the observed cytological and biochemical effects of Guarana, it may be subjected to a thorough evaluation of toxicity before it is available for human use.