We tested two hypotheses: 1) that the spontaneous enhancement of phrenic motor output below a C2 spinal hemisection (C2HS) is associated with plasticity in ventrolateral spinal inputs to phrenic motoneurons, and 2) that phrenic motor recovery in anesthetized rats after C2HS correlates with increased capacity to generate inspiratory volume during hypercapnia in unanesthetized rats. At 2 and 4 weeks post-C2HS, ipsilateral phrenic nerve activity was recorded in anesthetized, paralyzed, vagotomized and ventilated rats. Electrical stimulation of the ventrolateral funiculus contralateral to C2HS was used to activate crossed-spinal synaptic pathways phrenic motoneurons. Inspiratory phrenic burst amplitudes ipsilateral to C2HS were larger in the 4 vs. 2 week groups (p<0.05); however, no differences in spinally evoked compound phrenic action potentials could be detected. In unanesthetized rats, inspiratory volume and frequency were repeatedly quantified using barometric plethysmography at inspired CO₂ fractions (FICO₂) between 0.0 and 0.07 (FIO₂=0.21, balance N₂) prior to, and 2, 3 and 5 weeks post-C2HS.Inspiratory volume was diminished, and frequency enhanced, at 0.0 FICO₂ (p<0.05) 2-weeks post-C2HS; further changes were not observed in the 3 and 5 week groups. Inspiratory frequency during hypercapnia was unaffected by C2HS. Hypercapnic inspiratory volumes were similarly attenuated at all time points post-C2HS (p<0.05), thereby decreasing hypercapnic minute ventilation (p<0.05). Thus, increases in ipsilateral phrenic activity during 4 weeks post-C2HS have little impact on the capacity to generate inspiratory volume in unanesthetized rats. Enhanced crossed phrenic activity post-C2HS may reflect plasticity associated in spinal axons not activated by our ventrolateral spinal stimulation.