The abundance of the nitrogen isotopes in several source of nitrogen has been determined. In atmospheric nitrogen the absolute ratio, N/sup 14//N/sup 15/ , is 272.0 plus or minus 0.3. Small variations were observed for various sources of compressed gas, but the isotopic composition of the nitrogen isotopes of the atmosphere was constant (to 1 part in 7000) in samples collected at defferent geographical sites and altitudes above these sites. A procedure for the determination of the absolute abundance by dual collection of the 28/sup +/ and 29/sup +/ ion currents is given along with a method of circumventing the ever- present background problem in nitrogen abundance determinations. Proper procedures to be employed when assaying enriched nitrogen preparations and samples containing normal nitrogen mixed with another gas such as oxygen are described. (auth)