Adult male albino rats were orally administered 0, 25, 50 and 100 ppm of lead nitrate, mercuric chloride and cadmium chloride for 60, 120 and 180 days. The plasma sodium levels were decreased in rats exposed to varying doses of lead and mercury up to 180 days, while animals which consumed cadmium chloride showed an increase in sodium levels. In lead and mercury treated animals, plasma potassium levels were increased up to 180 days. The levels were decreased in cadmium exposed rats. These observations suggest that chronic exposure to these heavy metals considerably influences plasma sodium and potassium levels depending on the dose and duration of exposure.