Using theR(30) N2O laser line in order to saturate the infrareds a Q(9, 6, 9) (v2= 0 ↔v2= 1) vibration‐rotation line of NH3, the inversion of the microwaveJ= 9,K= 6,M= 9,v2= 0, has been obtained. Exact coincidence between the laser line and the infrared NH3transition is realized by Stark‐effect tuning. The variation of the Stark field through the exact coincidence permits studying the evolution of the microwave line versus the frequency mismatch of the pumping radiation.