Biotransformation of NO, nitrite and nitrate was investigated in rats and mice in a ' 5 NO inhalation experiment and intraperitoneal injection experiments of 5 N-nitrite and 5 5 N-nitrate, and the following results were obtained:
( 1) Rats were forced to inhale 5 NO ( 145 ppm, 123 minutes) or were given an intraperitoneal injection of ' 5 N-nitrite ( 2 mg animal -' as ' 5 N) or 15 N-nitrate ( 2 mg animal-' as 15 N), and determination of 15 N recovery in urine was made up to 48 h later The results were 55, 53 and 78 % of the inhaled or injected 15 N, respectively.
( 2) 15 N-nitrate in the urine was converted into a 6-nitro derivative of 3,4xylenol and its identification and quantitative determination were made by the GC-MS method As to 15 N-urea in the urine, identification and quantitative determination were made by the urease method 1 5 N was present in the urine of rats after 15 NO inhalation in the form of NO 3 and urea 75 and 24 % respectively In the urine of rats injected with 'SNnitrite, about 20 % of unidentified 15 N-compounds not discovered in the inhalation experiment was found The content of 15 N-urea in the urine after injection with 15 N-nitrate was lower than that after injection with 15 N-nitrite. ( 3) When "SN-nitrite ( 0 617 mg animal -' as 15 N) was injected intraperitoneally in mice, 60 7, 7 8 and 0 3 % of the injected 15 N were found in the urine, feces and exhaled gas (NO, NO 2 and NH 3 in the gas were caught) up to 48 h after injection respectively, and 1 6 % was found in the body 48 h after injection, but the remaining 30 % of 5 N could not be recovered.