The effect has been investigated of dietary lipids on the whole-body retention and organ distribution of organic and inorganic mercury in mice. A single oral dose of methylmercury chloride or mercuric chloride labelled with 203Hg was given to female NMRI mice fed semi-synthetic diets containing varying amounts (5, 10, 20 or 50%) of energy derived from lipid (coconut oil, soya oil, or cod liver oil). The whole-body retention and relative organ distribution of mercury depended on diet composition. Thus, a significant reduction of the whole-body retention of mercury was seen in mice fed a diet containing 50% cod liver oil compared with mice fed a diet containing 50% coconut oil. After oral administration of mercuric chloride the relative deposition of mercury in the kidneys increased while that in the liver decreased with increasing concentrations of soya oil or coconut oil in the diet. The whole-body retention of mercury after treatment with methylmercury chloride was significantly decreased in mice fed cod liver oil compared with mice fed coconut oil; there was no difference between mice fed cod liver oil and those fed soya oil. The relative disposition of mercury was significantly higher in all organs of mice fed a diet containing 20% energy from cod liver oil compared with mice fed a diet containing 20% energy from soya oil. The present study demonstrates that diet composition is of major importance to the toxicokinetics of methylmercury and mercuric mercury.