The alkaloid Punarnavine isolated from the plant Boerhaavia diffusa Linn. was studied for its anti-metastatic activity using B16F-10 melanoma cells in C57BL/6 mice. Administration of Punarnavine (40mg/kg body weight) prophylactically (95.25%), simultaneously (93.9%) and 10 days after tumor inoculation (80.1%) could inhibit the metastatic colony formation of melanoma in lungs. Survival rate of the metastatic tumor-bearing animals were increased significantly by the administration of Punarnavine in all the modalities compared to the metastasis bearing untreated control. These results correlated with the biochemical parameters such as lung collagen hydroxyl proline, uronic acid, hexosamine, serum sialic acid, serum gamma-glutamyltranspeptidase and serum vascular endothelial growth factor (VEGF) levels and histopathological studies. Punarnavine administration could suppress or down regulate the expression of MMP-2, MMP-9 (MMP--matrix metalloproteinase), ERK-1, ERK-2 (ERK--extracellular-signal-regulated kinase) and VEGF in the lung tissue of metastasis-induced animals. Punarnavine could inhibit MMP-2 and MMP-9 protein expression in gelatin zymographic analysis of B16F-10 cells. These results indicate Punarnavine could inhibit the metastatic progression of B16F-10 melanoma cells in mice.