The abundance of the nitrogen isotopes in several sources of nitrogen have been determined. In atmospheric nitrogen the absolute ratio, N¹⁴/N¹⁵, is 272.0 ± 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 different geographical sites and altitudes above these sites.