An extended but tractable model is developed to describe population transfer between the laser-excited state (e) and manifolds of higher-lying (II) and lower-lying (I) states of a metal atom in a flame. The conditions for partial Boltzmann equilibrium in II and for over-population of state e with respect to II are reformulated in a physically consistent manner. The model is used to interpret intensity ratios and saturation curves, measured for various Na-fluorescence lines in an H 2-O 2-Ar flame at 1 atm when the 5S-state is excited by two-photon absorption in a dye-laser beam with 1 μs pulse duration. Conclusions are drawn about the effective rate constants for collisional population transfer between 5S and the manifolds I and II. Existing models and previous experiments in similar systems are critically re-evaluated.