The transmission of a C.W. laser beam focussed in a high-temperature cell containing sodium and a noble gas is found to exhibit oscillations with a time constant of the order of 1 min.The observation of the fluorescence suggests that a laser-induced plasma is created during the phase of large absorption. We think that the effect can be related to the intrinsic bistable character of this system.We have observed that the transmission of a C.W. laser beam through a high-temperature (T= 270 "C) cell filled with sodium and a noble gas exhibits oscillation with a long-time constant of the order of (1 + 10) min. This effect is very spectacular since the transmission