The details of the hole-burning process are studied experimentally and theoretically in a Doppler-broadened gain or absorption line in pure CO2and in CO2-N2mixtures contained in a test cell. The change of absorption or gain of the test cell when irradiated by a saturating laser in a transition is probed with another laser operating in one of many transitions. In this way one may determine the rate of velocity cross relaxation within a single rotational-vibrational level and the rate of cross relaxation between different rotational levels of the upper and lower vibrational states. When the probing laser shares a common upper or lower level with the saturating laser, a pip is observed in the differential absorption or gain profile at total pressures of less than 0.5 torr. At higher pressures the relaxation across the velocity profile eliminates the pip. An analysis is presented of the population distribution in the multilevel system of CO2. Three of the relaxation rates (describing the relaxation across the velocity profile of a single level, the relaxation among rotational levels of the vibrational state, and the phenomenological relaxation rate of entry into and departure from the upper and lower vibrational states) are determined. The last of the three rates is found to be dominated by diffusion at the pressures used in the experiment.