The interactions among opioid-sensitive nociceptive modulatory systems, which include the midbrain periaqueductal gray, rostral ventromedial medulla and spinal cord, are likely to play a central role in the potent antinociception that results when morphine is administered systemically. The aim of the present study was to investigate the mechanisms through which local application of morphine, either in the periaqueductal gray or at the lumbar spinal cord in the rat, influences the activity of one population of putative nociceptive modulatory neurons in rostral ventromedial medulla, i.e. "on-cells". Previous studies have shown that the spontaneous and tail-flick-related firing of on-cells is invariably depressed when morphine is given systemically in doses demonstrated to inhibit the tail-flick reflex, and that a similar depression of this activity is produced when morphine is applied directly in the periaqueductal gray or intrathecal space. In the present experiments, on-cells were activated pharmacologically using iontophoretically applied glutamate to provide an indication of whether morphine-induced suppression of on-cell firing reflected a postsynaptic inhibition or a disfacilitation resulting from blockade of an excitatory input to the on-cell. Microinjection of morphine into the periaqueductal gray blocked glutamate-evoked activity of on-cells in parallel with its suppression of the tail-flick reflex, suggesting activation of an inhibitory input to these cells. No change in glutamate-evoked activity occurred in rats in which morphine did not produce antinociception. Intrathecal administration of morphine did not alter the glutamate-evoked activity of these neurons despite blocking the tail-flick reflex, suggesting that morphine acting in the spinal cord removes an excitatory input to on-cells.(ABSTRACT TRUNCATED AT 250 WORDS)