Abstract. For m = 1, 2, 3, we consider differential systems of the formwherefunctions, and T -periodic in the first variable, being D an open subset of R n , and ε a small parameter. For such system we assume that the unperturbed system x = F 0 (t, x) has a k-dimensional manifold of periodic solutions with k ≤ n. We weaken the sufficient assumptions for studying the periodic solutions of the perturbed system when |ε| > 0 is sufficiently small.