The authers studied the excretion of N
15 in two cases after administering orally the N
15 labeled ammonium citrate(42.3 atomic % excess). One case was a healthy male infant of 3 and 3/4 years old with the body weight of 12.9 Kg and 373.2 mg(25.37 meq as N
15 )was administered orally. The excretion of administered N
15 in the urine reached a state of saturation in about 2 days and was approximated by an exponetial function. From the analysis of the results, it was estimated that roughly about 65% of the administered N
15 was excreted in about 3 days. The excretion rate of N
15 may be analysed into two portions: one is a rapidly excreted urea cycle with a half life of about5hours, while another the other slow metabolic pathway or pool with a half life of about 36 hours. The total nitrogen excretion was roughly approximated by the synthesis of the two functions representing the rapid and the slow metabolic route. Another case was a male infant of 5 and 5/12 years old with the body weight of 16 Kg, who was suffering from nephrosis, and 473 mg(31.57 meq as N
15)of N
15 labeled ammonium citrate was administered orally. Almost all of the administered N
15 was excreted in about 2 days and 78% of the administered N
15 was excreted in urine during 4 days. Its excretion rate was approximated by an exponential function with the same tendency as in the case of urea which was reported previously in the preliminary note. The half excretion time was about 5 hours. The measurement of the N
15 atomic % was carried out by Prof. Hayakawa of Osaka Prefectural University with the Hitachi Mass Spectrometer of RM-C Type.
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