Experiments were designed to establish whether mercuric chloride administered to rainbow trout (Salmo gairdneri Rich.) through the gills or by way of the mouth is converted in vivo to methylmercury. The mercury was measured by atomic absorption spectrophotometry and methylmercury determinations were performed by extraction and gas chromatography. The experiments show that while mercuric chloride administered by either method increased the total mercury content of kidney, liver, intestine, and muscle tissues, no evidence was found to support the hypothesis that mercuric chloride was methylated in fish to form methylmercury.