Distribution of a single oral dose of methyl mercury (10 mg Hg/kg body weight) was followed from 90 min up to 120 h in plasma, erythrocytes, and brain of Japanese quails and rats. Significantly higher Hg concentrations were observed in plasma and brain of quails and red blood cells of rats. Blood/brain ratio decreased in quails from 6 to 2 at 24 h and 120 h respectively, whereas it increased in rats. Erythrocyte/plasma ratio in quails was about three times lower and averaged 54. The differences in Hg distribution were accompanied by a more than 3-fold higher acute toxicity in quails under adequate experimental conditions.