It has now been established that functional recovery after spinal cord injury (SCI) depends on several parameters, including animal strain. Here we demonstrate that rats from the same strain (Wistar) but from two independent commercial suppliers present different motor, sensory, and autonomic outcomes after a standard model of SCI, the so-called compression model. Recovery is correlated with the extension of the lesion, and we show that the vertebral canal diameter varies between the two suppliers. To substantiate this point, we carried out another set of experiments, with the so-called contusion model, which requires bone ablation and thus whose extension is not related to vertebral canal diameter. We show that there is no difference between the two suppliers. The purpose of our communication is to alert researchers on how crucial it is to control experimental parameters as closely as possible and to establish a standard for animal experiment in order to avoid unexpected biases. © 2008 Wiley-Liss, Inc.