The effect of piperine on the inhibition of lung metastasis induced by B16F-10 melanoma cells was studied in C57BL/6 mice. Simultaneous administration of the compound with tumor induction produced a significant reduction (95.2%) in tumor nodule formation. Increased lung collagen hydroxyproline (22.37 microg/mg protein) in the metastasized lungs of the control animals compared to normal animals (0.95 microg/mg protein) was significantly reduced (2.59 microg/mg protein) in the piperine-treated animals. The high amount of uronic acid (355.83 microg/100 mg tissue) in the metastasized control animals was significantly reduced (65 microg/100 mg tissue) in the animals treated with piperine. Lung hexosamine content was also significantly reduced in the piperine-treated animals (0.98 mg/100 mg lyophilized tissue) compared to the untreated tumor-bearing animals (4.2 mg/100 mg lyophilized tissue). The elevated levels of serum sialic acid and serum gamma glutamyl transpeptidase activity in the untreated control animals was significantly reduced in the animals treated with piperine. The piperine-treated animals even survived the experiment (90 days). Histopathology of the lung tissue also correlated with the lifespan of the drug-treated animals. Our results demonstrate the antimetastatic activity of piperine, an alkaloid present in plants such as Piper nigrum and Piper longum.