According to the stochastic self propagating star formation theory (SSPSF, Gerola and Seiden, 1978), the star formation process in galaxies changes from a fluctuating but continuous mode to a bursting one, when the size of the system becomes relatively small (R ≤ 3 Kpc). Then, due to its size, the LMC should be a system undergoing fluctuating but continuous star formation activity, whereas the SMC should be in the region between continuous and bursting modes (Gerola et al. 1980). In order to look better inside this problem a model of chemical evolution of the two Clouds, which takes into account the stochastic star formation rate, has been built. For the SMC both the continuous and the bursting modes of star formation have been considered. As a result we find that the different chemical histories of the two Clouds may be related to the fact that SMC has undergone several bursts of star formation (between 50 and 60), while a continuous star formation activity was present in LMC.