Chemical activation of upper cervical spinal neurons modulates activity of thoracic respiratory interneurons in rats. The aim of the present study was to examine the effects of chemical activation of C₁-C₂ spinal neurons on thoracic spinal respiratory motor outflows. Electroneurograms of left phrenic (n=23) and intercostal nerves (ICNs, n=93) between T₃ and T₈ spinal segments were recorded from 36 decerebrated, vagotomized, paralyzed and ventilated male rats. To activate upper cervical spinal neurons, glutamate pledgets (1 M, 1 min) were placed on the dorsal surface of the C₁-C₂ spinal cord. Glutamate on C₁-C₂ increased ICN tonic activity in 56/59 (95%) ICNs. The average maximal tonic activity of ICN was increased by 174% (n=59). After spinal transection at rostral C₁, glutamate on C₁-C₂ still increased ICN tonic activity in 33/35 ICNs. However, the effects of C₁-C₂ glutamate on ICN phasic activity were highly variable, with observations of augmentation or suppression of both inspiratory and expiratory discharge. C₁-C₂ glutamate augmented the average amplitude of phrenic burst by 20% whereas the increases in amplitude of ICN inspiratory activity, when they occurred, averaged 120%. The burst rate of phrenic nerve discharge was decreased from 34.2±1.6 to 26.3±2.0 (mean±SE) breaths per min during C₁-C₂ glutamate. These data suggested that upper cervical propriospinal neurons might play a role in descending modulation of thoracic respiratory and non-respiratory motor activity.