Measurements of the emission from Pb+in the afterglow of a pulsed He : Pb discharge in a large‐bore tube and of the intensities of Pb+and Pb emission lines in a small‐bore dc discharge tube have been made to clarify the excitation mechanism of the He : Pb laser. In both He :  pure Pb and He : Pb containing a low concentration of Cd to act as a probe, the rate constants for the decay of the Pb+and Cd+emission lines obtained using the large‐bore tube indicated that the only excitation mechanism was thermal charge exchange between2S1/2He+and ground‐state Pb atoms. The collision cross section for the He+‐Pb reaction was determined to be 4.0×10−16cm2from measurements of the variations of the decay rates of the 5608.8‐A Pb+, 4386.4‐A Pb+, and 5378‐A Cd+emission lines with Pb concentration. The variation of the rate constant of the 4416‐A Cd+line indicated that the collision cross section for the 2s3S1He‐Pb Penning ionization is 14.8×10−16cm2. In a small‐bore tube, Penning ionization was verified to be the dominant excitation mechanism for the 4245‐A˚ Pb+emission line but not the 5608‐A˚ Pb+laser line. The change in the relative importance of the various excitation mechanisms with the discharge‐tube diameter, and its ramifications regarding other metal‐vapor laser systems, is discussed.