The effect of arsenic on the binding of metronidazole (an anti-amoebic agent) to bovine serum albumin (BSA) was studied by equilibrium dialysis (ED) method in order to justify the nature of binding capability of the drug and arsenic to BSA. There was decrease in metronidazole concentration due to addition of arsenic. But the free metronidazole concentration was increased with increasing the metronidazole concentration when only the BSA was present. The result obtained when no arsenic was added the free concentration of metronidazole was 41% whereas this release was only 7% when arsenic was added with an increasing concentration from 0.5x10-5 to 10x10-5M. But the free concentration of metronidazole was 36% to 71% when only metronidazole was added to BSA from 0.5x10-5 M to12x10-5M. This result suggests that in presence of arsenic, metronidazole shows a greater binding to BSA and results in a decreased serum drug concentration. It could be suggested that in the presence of arsenic, metronidazole may form complex with arsenic, arsenic may increases the binding affinity to its sites or may form complex to BSA and metronidazole.