1. Descending influences produced by focal electrical stimulation in the nuclei reticularis gigantocellularis (NGC) and gigantocellularis pars alpha (NGC alpha) on spinal nociceptive transmission and the dorsoventral region of spinal white matter mediating stimulation-produced modulation were examined in pentobarbital sodium-anesthetized, paralyzed rats. Spinal units studied responded to mechanical stimuli and noxious heating (50 degrees C) of cutaneous receptive fields confined to the glabrous skin of the ipsilateral hind foot. Recording sites were located in laminae I-VI of the L3-L5 spinal segments. 2. Electrical stimulation in the NGC or NGC alpha produced both facilitation and inhibition of responses of spinal units to noxious heating of the skin. At 33 of 57 stimulation sites in the NGC and NGC alpha, electrical stimulation produced biphasic effects, facilitating responses at lesser intensities (5-25 microA) and inhibiting responses at greater intensities (50-100 microA). At 21 other sites in the NGC and NGC alpha, electrical stimulation (5-100 microA) only inhibited, and at 3 sites stimulation (5-100 microA) only facilitated responses of spinal units to noxious heating of the skin. 3. Electrical stimulation in the NGC or NGC alpha contralateral to the spinal recording site produced the same magnitude of facilitation/inhibition or inhibition of spinal nociceptive transmission as did stimulation in the ipsilateral NGC and NGC alpha. 4. The latencies to descending facilitation and inhibition of spinal nociceptive transmission from the NGC and NGC alpha were estimated by a cumulative sum technique to be 232 and 80 ms, respectively. 5. Responses of spinal units to graded heating (42-50 degrees C) of the skin exhibited positively accelerating stimulus-response functions (SRF) throughout the temperature range tested. Electrical stimulation at lesser, "facilitating" intensities produced a parallel, leftward shift of the SRF, whereas stimulation at greater, "inhibitory" intensities significantly decreased the slope of the SRF without affecting the threshold for response. 6. To determine whether activation of cell bodies in the NGC or NGC alpha were capable of replicating the effects of electrical stimulation, L-glutamate was microinjected into sites where electrical stimulation facilitated at lesser and inhibited at greater intensities the responses of spinal units to 50 degrees C heating of the skin. L-Glutamate (5 nmol) produced a rapid onset, short-lasting and reproducible facilitation of nociceptive transmission; glutamate microinjection into the same site at a greater dosage (50 nmol) inhibited responses of the same spinal units.(ABSTRACT TRUNCATED AT 400 WORDS)