This paper deals with decomposition techniques for nonlinear large ndash scale stochastic systems which have the feature that the interactions between the various subsystems are nonadditive. We consider the system differential equations into a hierarchical form (see Vidyasagar §). We prove that the overall system is exponentially stable (respectively stabilizable) in mean square if and only if each of the subsystems is exponentially stable (respectively stabilizable) in mean square. Several examples are presented to illustrate the various theorems