A study of the transient natural convection in a square cavity (A = 1) is conducted numerically. The system is cooled isothermally from the above and partially heated from the side with two types of excitations. In the first type, the temperature is varied sinusoidally in time while in the second, it varies with a pulsating manner. For each type of excitation, the effect of the amplitude of the excitating temperature (0 less than or equal to a less than or equal to 1), its period (10(-5) less than or equal to tau less than or equal to 4), the length of the heated element (0 less than or equal to B = l＇/L＇ 1), its position (-0.25 less than or equal to epsilon = (y(2)＇ + y(1)＇-H＇)/(2H＇) less than or equal to 0.25) and the Rayleigh number (10(3) less than or equal to Ra less than or equal to 10(6)) on the resulting fluid flow and heat transfer is studied. The working fluid is air (Pr = 0.72). The results obtained show that the mean values of heat transfer and flow intensity are considerably different with those obtained in stationary regime. In addition, it is demonstrated that, in some conditions, the heat evacuated through the cold surface of the cavity resonates with the imposed temperature. The frequencies for which this phenomenon is observed are calculated for the two types of excitations considered in this study.