Abstract 1. This study concerns the effects of mercuric chloride (MC) and methylmercuric chloride (MMC) on the activity of mucous cells in the skin and gill epithelium of rainbow trout. 2. Of the three parameters used to measure this activity, namely density of mucous cells, mucus co content of the tissues, and mucus release into the water, the latter appeared to be the most sensitive to treatment with mercury. 3. When the two compounds are used in equal concentrations of mercury MC exerts a much stronger effect on mucus secretion than MMC. 4. Possible consequences of mercury-induced enhancement of mucus production for the respiratory and osmoregulatory functions of the gills are discussed.