The toxicity of mercury to animals and man is well established and this depends greatly on the form of the mercury compounds. In most animals' species, including man, the kidney is the main site of deposition of inorganic mercury and target organ for its toxicity. In the present study Spirulina fusiformis (a cyanobacterium, belongs to family--Oscillatoriaceae) has been investigated as a possible modifier of mercury induced renal damages in Swiss albino mice. Animals were divided into four groups. (i) Control group--only vehicle (0.9% NaCl) was administered as i.p. (ii) HgCl(2) treated group--5.0 mg/kg b.wt. HgCl(2) was administered as i.p. (iii) Spirulina treated group--800 mg/kg b.wt. Spirulina extract was administered orally. (iv) Combination group--S. fusiformis was administered 10 days before mercuric chloride administration and continued upto 30 days after mercuric chloride administration (5.0 mg/kg b.wt.). The animals were autopsied on 1, 3, 7, 15 and 30 days after treatment and the activity of alkaline phosphatase (ALP), acid phosphatase (ACP), lactate dehydrogenase (LDH) and MDA (malondialdehyde) level were measured in kidney homogenates. The results indicated that there was a time-dependent significant enhancement in MDA content and ACP activity and decrease in LDH and ALP activity observed after HgCl(2) treatment. Mercury intoxication also induces pathological alterations in the kidney such as degeneration of glomerulus, proximal and distal tubules. A dose-dependent mortality was also observed following administration of different doses of HgCl(2). In combined treatment of Spirulina with HgCl(2), a significant decrease in MDA content and ACP activity and elevation in LDH and ALP activity was observed as compared to HgCl(2) treated group. Spirulina pre- and post-treatment with mercury also significantly reduces pathological alterations in kidney. Thus, the results from the present study suggest that S. fusiformis can significantly modify the renal damages against mercuric chloride induced toxicity.