The intravenous (i.v.) administration of serotonin (5-HT) to lightly pentobarbital-anesthetized rats is known to produce a triad of reflex cardiovascular responses, distinct afferent-mediated pseudaffective reactions, and a vagally mediated inhibition of the nociceptive tail-flick (TF) reflex consistent with 5-HT acting as a noxious stimulus. In the present experiments we examined the involvement of capsaicin-sensitive afferents in mediating these responses. Lightly pentobarbital-anesthetized 16-week-old male Sprague-Dawley rats which had been treated as neonates (in the first 48 h of life) with capsaicin (50 μg/kg. s.c.) were compared to age-matched neonatal vehicle-treated controls. Whereas the i.v. administration of 5-HT produced a dose-dependent (6-96 μg/kg, i.v.) inhibition of the nociceptive TF reflex (ED50 = 48.1 ± 113 μg/kg; n = 7) and distinct pseudaffective responses (usually by 24-48 μg/kg) in vehicle-treated rats. 5-HT (6-192 μg/kg, i.v.) failed to significantly alter TF latency or produce pseudaffective behaviors in the capsaicin-treated rats (n = 10). There was no difference in baseline TF latencies between the two groups. There were essentially no differences between vehicle- and capsaicin-treated rats with respect to the initial cardiopulmonary vagal afferent-mediated (Benzold-Jarish reflexes) decrease in heart rate and arterial blood pressure or the subsequent pressor phase. However, the magnitude of the late hypotensive phase was significantly greater in capsaicin-treated rats. Capsaicin-treated rats were significantly heavier than vehicle-treated rats (482.0 ± 12.4 g vs 439.3 ± 12.1. g) and also exhibited small cutaneous lesions around the nostrils, on the skin in and around the vibrissae, behind and on the back of the ears, on the throat, and on the underside of the belly. Radioimmunoassay showed that there was a significant (P < 0.05) depletion of substance P (58.3%) and calcitonin gene-related peptide (50.7%) in the lumbar spinal cord of neonatal capsaicin-treated rats (n=8) compared to vehicle-treated rats (n=5). These results demonstrate that the 5-HT-induced, vagally-mediated inhibition of the TF reflex and pseudaffective responses are mediated by capsaicin-sensitive afferents. This strongly suggests that (1) afferent fibers subserving the nociceptive response are predominantly unmyelinated C-fiber vagal afferents and (2) these afferents are different from those vagal afferents that mediate the cardiovascular responses to i.v. 5-HT. © 1991.