An experimental investigation was made to determine in detail an NMR ground state transition lineshape function of a two‐level system (spin 1/2). The optically pumped vapor of199Hg was selected as an investigative medium because of the narrow bandwidth (∼1 Hz) and large maximum signal‐to‐noise power ratio (>60 dB at an observation bandwidth of 1 Hz) of the associated NMR. The shape at 1.0 kHz, under the conditions of the experiment, for199Hg was found to be describable within experimental error over a 750 linewidth spectral region using only simple combinations of Lorentzian functions. In addition, the saturation properties were found to agree with the prediction of modified Bloch susceptibility equations.