The present study investigates response and stability characteristics of gear-pair systems supported on oil journal bearings. The systems examined are driven by a motor providing a known torque, which is transferred to a load element through shafts and a gear-box. Here, the emphasis is placed on the gear-pair action, by assuming that the interconnecting shafts are rigid. This results in mechanical models with strongly nonlinear characteristics, whose long time dynamics are then effectively studied by applying appropriate numerical methodologies. In this way, useful information is obtained for the influence of the loading, the gear mesh and the journal parameters on the system dynamic behavior, including periodic, quasiperiodic and chaotic response. Efforts are also directed towards predicting the onset of oil whirl instability and investigating its effects on the gear-pair dynamics.