Dietary interactions between methylmercury (Ch3Hg) and sodium selenite (Na2SeO3) were studied in Japanese quail. Addition of 0.35-6 p.p.m. Se to diets containing toxic levels of Hg (5-30 p.p.m. Hg) reduced the toxicity of methylmercury and increased the survival of Japanese quail. The survival increased with increasing levels of Se in the diet. Selenium as 6 p.p.m. did not cause any mortality in males and caused only relatively low mortality in females. However, 12 p.p.m. Se depressed the survival of quail, especially females. No consistent effect of Hg, Se or both was observed on body weight or feed consumption. High levels of Hg reduced egg production, fertility and hatchability, and the addition of Se lessened these effects of Hg. Selenium in the diet alone (6 or 12 p.p.m.) generally produced lower hatchability and a high percentage of deformed embryos, and 12 p.p.m. Se also depressed egg production. Addition of 5-15 p.p.m. Hg to such Se diets overcame these effects and reduced the percentage of abnormal embryos more than 50%. Analysis of tissues for total Hg showed that Hg was distributed in a pattern typical for alkyl mercurials. Hg levels in Se-protected birds equaled or exceeded the levels in those fed Hg without Se. Se levels in tissues were generally elevated by feeding Hg, especially in brain. Extremely high brain Hg levels, up to 58 p.p.m., were observed in birds fed 15 p.p.m. Hg plus 6 p.p.m. Se for 20 weeks prior to sacrifice. The levels of Se in the brain of these birds was elevated (4 p.p.m.), but not equimolar to Hg.