Abstract. In this paper, using Leray-Schauder degree arguments, critical point theory for lower semicontinuous functionals and the method of lower and upper solutions, we give existence results for periodic prob-r dt = 0. In particular we show that in this case we have non-resonance, that is periodic problemhas at least one solution for any continuous function e : [0, T ] → R. Then, we consider Brillouin and Mathieu-Duffing type equations for which r(t) ≡ b1 + b2 cos t and b1, b2 ∈ R.Mathematics Subject Classification (2010). 34B15, 34B16, 34C25, 35J20, 35J60.