SummaryThe oral LD 5 so of Na NO 2 for rats was found to be O 15 g per kg body weight The methemoglobin level increased to 45-80 %, 1 h after the administration of the LD 5 o dose and returned to the normal level after 24 h. The dose-maximum methemoglobin concentration curve was found to be Sshaped Formation of nitrosyl hemoglobin preceded that of methemoglobin, its maximum concentration being a quarter of that of the latter derivative In rats receiving 0 5 % Na NO 2 as drinking water, the concentration of methemoglobin showed a characteristic daily change ( 4 to 88 %) due to the circadian rhythm of the animal in drinking After 6 months, slight nitrosyl hemoglobin production, Heinz body formation, anisocytosis, and hypohemoglobinemia were observed.