As it is well known, the dynamics of the stochastic SIRS epidemic model with mass action is governed by a threshold R S . If R S < 1 the disease dies out from the population, while if R S > 1 the disease persists. However, when R S = 1, classical techniques used to study the asymptotic behaviour do not work any more. In this paper, we give answer to this open problem by using a new approach involving some adequate stopping times. Our results show that if R S = 1 then, small noises promote extinction while the large one promote persistence. So, it is exactly the opposite role of the noises in case when R S = 1.2010 Mathematics Subject Classification. 92B05, 60G51, 60H30, 60G57.