Following recent reports on strong uv and ir laser transitions in noble-gas mixtures, we attempted to determine the dominant precursor reactions leading to laser action. In this paper we present experimental results of time-dependent spectroscopy of the various species existing in He/Xe mixtures excited by fast transverse electrical discharges. We also present some parametric measurements of the He/Xe laser in the ir region. A plasma model is suggested for explaining the time behavior of the helium and xenon emissions. According to this model, which is justified by the experimental results, ir laser action in the He/Xe mixture is excited mainly by energy transfer from metastable helium atoms and molecules by Penning ionization of xenon atoms.