The histopathological studies are useful in evaluating the pollution potential of toxicants since trace levels of chemicals do not bring about animal mortality over a given period and are capable of inducing considerable damage to organ. An investigation was carried out in the vital tissue - Gill of Mugil cephalus inhabiting in the Uppanar estuary, Cuddalore at three different station for the period of 12 months. The toxic substances (heavy metals) in the effluent was maximum at station II and III than at station I. The pathological changes were severe at station II and III then at station I.