Growing and laying chickens were fed practical-type diets containing 0, .05, .15, .45, and 1.35 ppm of mercury as methyl mercuric chloride. Chicks that had been fed mercury throughout life retained 61, 82, 91, and 95% of the mercury ingested from the respective diets at 8 weeks of age. The half-retention times of mercury, following withdrawal of dietary mercury after the 8-week period, were 8.4 and 23.4 days, respectively, for liver and kidney in chicks fed 1.35 ppm of mercury. Half-retention times in kidney and liver decreased with lower mercury intake, i.e., with lower concentrations in the tissues at the time of mercury withdrawal. Adult laying birds, continuously fed diets containing methyl mercuric chloride, laid eggs with gradually increasing concentrations of mercury until plateau concentrations reflecting the respective dietary concentrations were reached. Upon withdrawal of mercury from the diet, mercury concentrations in the eggs laid by the birds fed .05, .15, .45 and 1.35 ppm declined to reach half of the concentrations at the time of withdrawal in 17, 13, 10, and 9 days, respectively. Likewise, the half-retention time of mercury in the liver, kidney, heart muscle, pectoral muscle, and brain tissue of the adult birds depended upon the tissue concentration at the time mercury was withdrawn from the diet and was inversely proportional to initial tissue concentration according to the equation: 1n y = 1.92 - .39 1n (7 x-5), where x is the initial tissue concentration in ppm and y is half-retention time in weeks, r = -.95.