Persistent organochlorines such as PCBs, DDTs and HCHs are ubiquitous contaminants found in seas and oceans worldwide. They are transferred from terrestrial contaminant sources to open ocean through atmosphere and ultimately deposit into the oceanic waters. The tropical regions are likely to contribute greatly to the global contamination of some organochlorines in recent years. The contaminants in water phase are taken into ecosystems following the prey-predator relationship in the food-chain. In this context, cetaceans like dolphins, porpoises and whales are recognized as one of the animal groups most at risk from persistent organochlorines in the environment. They accumulate extremely high concentrations of organochlorines through feeding, and transfer large quantities to offspring during lactation. Unfortunately, they have a small capacity to degrade these contaminants. It may be concluded that marine mammals, particularly cetaceans, are among the most vulnerable organisms to long-term toxicity of hazardous man-made chenicals.