In this paper we study the stability under stress of polymeric thin film transistors, PTFTs, fabricated with poly(methyl methacrylate) PMMA on poly(3-hexylthiophene) P3HT as dielectric and semiconductor layers, respectively. Devices were tested during and after gate bias stress combined with drain bias When. the stress is done at low drain bias, V-T shift in linear transfer curves remained below 1 V. When both high gate and drain bias are applied during stress, linear transfer curves shifted less than 5 V. In both cases hysteresis was less than 1 V. Transfer curves for V-DS = -30 V after stress showed deformation in addition to shift. Variation of I-DS at V-DS = -30 V during both types of stress remained always below 6%. Mechanisms responsible for the observed behavior are discussed. (C) 2009 Elsevier Ltd. All rights reserved.