A He&sngbnd;Ne laser heterodyne system was utilized to evaluate the dispersive effect at 3.39 &mgr; in a He&sngbnd;He laser. The dispersion introduced by lasering atoms at 3.39 &mgr; was measured as a function of cavity loss, excitation level, and deviation of the laser frequency from line center. Theoretical and experimental results were compared, and close agreement was obtained if the natural linewidth was taken to be 115 MHz and the Doppler linewidth was taken to be 315 MHz for the 3.39‐&mgr; transition.