1. In anesthetized rats recordings were made from neurons in the medulla caudal to the obex. In the medullary dorsal horn, typical trigeminal noxious specific, nonnoxious specific, and convergent neurons were found. In nucleus cuneatus, typical dorsal column units were recorded. In subnucleus reticularis dorsalis (SRD), recordings were made from neurons which exhibited convergence of nociceptive inputs from the entire body. In subnucleus reticularis ventralis (SRV) we recorded both from neurons with spontaneous activity that were either unaffected or inhibited by noxious stimuli applied to various parts of the body and from respiratory neurons. The present paper deals particularly with the nature of the stimuli that activated reticular neurons exhibiting nociceptive convergence. 2. Neurons with nociceptive convergence could be activated by mechanical, thermal, or chemical noxious stimuli applied to widespread areas of the body. By using percutaneous electrical stimulation, we found that they responded to the activation of peripheral fibers in the A delta- and C-range. Two neuronal subpopulations were defined according to the way in which SRD neurons responded to the electrical stimuli, namely: "neurons with total nociceptive convergence" (TNC) and "neurons with partial nociceptive convergence" (PNC). 3. The great majority (84%) of TNC neurons did not exhibit spontaneous activity and none of these neurons gave responses to heterosensory (flashes, whistle sounds) or proprioceptive stimuli. Most (88%) did not respond to any kind of innocuous cutaneous stimuli. By contrast, the entire population of TNC neurons responded to noxious mechanical, thermal, and visceroperitoneal stimuli. In the majority of cases (71%) long-lasting afterdischarges were observed following cessation of the application of the nociceptive stimulus. 4. All the TNC neurons responded to suprathreshold percutaneous electrical stimulation (2-ms duration) with two peaks of activation no matter which part of the body was stimulated. By stimulating two regions of the tail, 100 mm apart, we determined that the early and late peaks of activation were triggered by activities in peripheral fibers with mean conduction velocities of 10.8 +/- 0.5 and 0.74 +/- 0.05 (SE) m/s, respectively, i.e., A delta- and C-fibers. The mean thresholds for obtaining A delta-fiber components were found in the 0.4-0.7-mA range; the mean thresholds for obtaining C-fiber components were found in the 6-7.5- and 3-4-mA range for the face and the other parts of the body, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)