The median tolerance limit (MTL) for methyl mercury chloride at 24, 48, and 96 h was 0.084, 0.045, and 0.024 mg/liter as mercury, respectively, for fry; and 0.125, 0.066, and 0.042 mg/liter as mercury, respectively, for fingerlings. The MTL (24 h) for mercuric chloride for fingerlings was 0.90 mg/liter as mercury. Fingerlings exposed to methyl mercury chloride concentrated mercury in their tissues much more rapidly than did those exposed to mercuric chloride. The acute toxic action of both compounds was exerted on the gills. Mercuric chloride caused severe epithelial necrosis. Poisoning by methyl mercuric chloride was characterized by epithelial cell swelling and hyperplasia, a marked increase in the number of epithelial cells in mitosis, and terminal epithelial desquamation.