Population inversion by adiabatic rapid passage (ARP) utilizing narrow saturation resonances is observed for an infrared transition of NH3. The population change produced by sweeping the frequency of a strong saturating laser field through the center of a Doppler‐broadened absorption line is probed by a weak counterpropagating field as in a Lamb‐dip experiment. When the ARP conditions are satisfied, inversion of population is detected as amplification of the probe wave near the line center.