The do bias field. dependence of the dielectric loss in SrTi03 thin films deposited onto MgO substrate is investigated. The experimental data obtained at different frequencies of the ac field (8 and 16 GHz) from differently processed films (as deposited and oxygen annealed) strongly suggest the occurrence of a crossover in the dominant loss mechanism. The crossover is driven by the do bias field: at weak fields the loss is governed by an extrinsic mechanism(s) whereas, at higher fields, the contribution of an intrinsic mechanism (dc field-induced quasi-Debye loss) becomes predominant. (C) 2004 American Institute of Physics.