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Descending influences on the spinal nociceptive tail-flick (TF) reflex produced by focal electrical stimulation and glutamate microinjection in the nuclei reticularis gigantocellularis (NGC) and gigantocellularis pars alpha (NGCα) were examined and characterized in rats lightly anesthetized with pentobarbital. Both inhibition and facilitation of the TF reflex were produced by electrical stimulation at identical sites in the NGC/NGCα; glutamate microinjection only inhibited the TF reflex.The chronaxie of stimulation for inhibition of the TF reflex was 169 ± 28 μsec. Inhibition of the TF reflex by stimulation was produced throughout the NGC and NGCα; intensities of stimulation for inhibition were least in the ventral NGC and in the NGCα. At threshold intensities of stimulation, inhibition of the TF reflex did not outlast the period of stimulation.Facilitation of the TF reflex was produced at many of the same sites at which stimulation inhibited the TF reflex, but always at lesser intensities of stimulation (mean, 10 μA vs. 43 μA for inhibition, n = 25). Stimulation in the NGC/NGCα at threshold intensities for facilitation or inhibition of the TF reflex did not significantly affect blood pressure.Strength-duration characterization of electrical stimulation and microinjection of glutamate into identical sites in the NGC and NGCα suggest that descending inhibition of the TF reflex results from activation of cell bodies in the NGC and NGCα.