Far-infrared amplified emission from the v = 1 autoionizing levels of the ns (n = 9-14), np (n = 12- 15) and nf (n = 8-14) Rydberg states of NO is reported in the gas phase. The dominant emission process for nf is assigned as the nf > (n 1) g transition. Following the excitation of ns (n = 11-14), the emission from (n - 1) f energetically located above ns is detected. Following the excitation of np (n = 12 and 13), the emission from (n - 1) f is also detected. The absence of the emission from 11p leads to similar to 3 ps as a threshold lifetime of the upper state needed for amplification. (C) 2014 Elsevier B.V. All rights reserved.