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Zero-power gain measurements in a supersonic diffusion-type CW HF laser have been made using a pulsed HF probe laser. A peak zero-power gain of 10 percent/cm was measured on a vibrational-rotational transition and 7 percent/cm on a transition for flow rates corresponding to 1.8 kW of closed-cavity power. The variation of zero-power gain with rotational quantum number fits a simple rotational equilibrium model indicating increasing rotational temperatures with increasing distance from the nozzle exit plane. The model also indicates a total inversion for in for the transitions and a partial inversion further downstream.