Antinociception and cardiovascular responses produced by electrical stimulation in the nucleus tractus solitarius, nucleus reticularis ventralis, and the caudal medulla. In experiment 1, quantitative regional comparisons of the antinociceptive and cardiovascular responses produced by electrical stimulation in the caudal medulla, including regions such as the nucleus tractus solitarius (NTS), nucleus reticularis ventralis (NRV), nucleus reticularis gigantocellularis (NRGC), nucleus reticularis paragigantocellularis (NRPGC), nucleus raphe obscurus (NRO), and medial portions of the lateral reticular nucleus (LRN), were made in the rat. Electrical stimulation in all of these regions resulted in inhibition of the nociceptive tail-flick reflex, although the threshold intensity for inhibition was greater for sites in NTS compared to many sites ventral to the NTS. Antinociception was generally accompanied by an increase in mean arterial blood pressure, with the exception of sites in the NRO, where depressor responses were evoked by stimulation. Detailed comparisons between the NTS and NRV revealed that greater intensities of electrical stimulation were required to produce antinociception for sites in the NTS as compared to the NRV. There were no significant differences in threshold intensities for antinociception as a function of rostrocaudal subdivisions of the NTS, but the lateral subdivision of the NTS was significantly more efficacious than the medial subdivision. This mediolateral difference within NTS was primarily due to stimulation in medial sites producing overt movements in some animals, probably due to stimulation of adjacent midline nuclei or pathways. Within the NRV, thresholds for inhibition of the tail-flick reflex were greater for sites in the dorsal subdivision as compared to the ventral subdivision, which contains spinopetal projections from the NRM. The slopes of the lines of recruitment for inhibition of the tail-flick reflex at stimulation sites in either the NTS or NRV were both very steep, similar to other forms of antinociception. In experiment 2, the pulse duration of electrical stimulation was varied for sites of stimulation in the lateral NTS and NRV to generate strength-duration curves. This experiment confirmed that stimulation sites in the lateral NTS required greater current intensities to inhibit the tail-flick reflex than sites in the NRV. However, the chronaxies derived from the strength-duration functions for the NTS or NRV were both approximately 170 microseconds, indicating that the antinociceptive effects in these regions may not be exclusively due to the stimulation of fibers of passage. These results are discussed in terms of the role of the NTS, NRV, and caudal medulla in the modulation of nociceptive responses and cardiovascular function. 