Rainbow trout (Salmo gairdneri) fingerlings were fed rations containing 4, 8, 16, and 24 ppm mercury as methyl mercury chloride over a 105-day period. Fish receiving the 16- and 24-ppm rations had significantly higher blood packed cell volumes than did controls. Hyperplasia of the gill epithelium was the only morphologic alteration detected in these fish. Individual fish accumulated up to 30 ppm mercury in muscle tissue, but no mortality that could be attributed to mercury occurred.The results suggest that trout can tolerate a large body burden of mercury, if this mercury is acquired over a period of time.