A numerical model of the excitation of ammonia in dense interstellar clouds predicts strong maser emission in various inversion doublet lines when infrared pumping to the lowest vibrationally excited state is included. Specifically the (6,3) transition mases, while the (5,3) and (4,3) transitions are in normal emission, as observed in W51. Model conditions are H 2 density between 10 8 and 10 10 cm −3 , NH 3 column density 10 18 cm −2 , and infrared temperature greater than 200 K, comparable with conditions required for OH and H 2 O masing in regions of star formation.