Rainbow trout (Salmo gairdneri) were exposed to 0.5 and 2.0 μg/g doses of ethyl-mercury (p-toluene sulfonanilide) “Ceresan” each day for a full year and 2.5 and 10 μg/g doses delivered every 5th day of feeding during the year. A further study extended dose levels from 5.0 to 25.0 μg/g Ceresan given daily. Exposure to the lower doses of mercury for either feeding schedule led to concentrations of mercury in muscle that were similar (regression slope equaled one) to the average daily index of dose for as long as 6 months of feeding. Assessment of concentration in muscle at 9 months of feeding showed a breakdown of the effect in all groups except the one receiving 0.5 μg/g of Ceresan daily. Both dose level and schedules influenced the concentrations of mercury in muscle. Daily treatment with higher doses, e.g., 5.0 through 25.0 μg/g, led to dose-related concentrations of mercury in muscle but the regression was greater than one. Orders of mercury concentration in a variety of other tissues differed significantly and were generally related to dose. Fish receiving 10.0 μg/g of mercury every 5 days or 5.0 μg/g or greater doses every day in their feed were unable, with few exceptions, to learn to avoid shock when preceded by a signal-light. But beyond performance on the learning task, there was no evidence of impairment of general behavior nor was there any indication of physical debilitation resulting from any treatment. There appeared to be a fairly rapid loss of mercury from selected tissues, although estimates of total body burden of mercury remained high after 6 months of mercury-free feeding.