The robustness problem of perturbed large-scalesystems is considered. The nominal large-scale system is stabilized by a local constant state feedback u,(t) = k,Xi(t) and the local optimal control law ui(t) = -R i -I BJ PiXi(t), respectively. In the above two cases, bounds are obtained for an allowable non-linear time-invariant (or timevarying) perturbation such that the resulting closed-loop large-scale system remains stable. The special case of a linear perturbation is also treated here. Nomenclature IR n real vector space of dimension n AT transpose of the matrix A A i ( A) ith eigenvalue of the matrix A II A lis spectral norm of the matrix A i.e. IIA lis = max [)'i(A T A)]112 , ( m )112 i.e. Ilgll = J,lgil 2 ,