  Consider the Lienard system $ u'' + f(u) u' + g(u) = 0$ with a center at the
origin 0. In the case where the period function $T$ is monotonic, we examine
periodic solutions of the perturbed equation $ u'' + a(u)u' + f(u) = \epsilon
h(t)$. {\it Key Words:} perturbed systems, Lienard equation, polynomial
systems.
