Along with well‐known data on the neurochemical mechanisms of nociceptor activation, there are still no clear data regarding changes in the cellular composition and morphological characteristics of spinal preganglionic neurons (SPN) after capsaicin treatment. The mechanism of capsaicin toxicity differs in developing and mature nerve cells. This study aimed to determine the number of SPN in the autonomic nuclei on spinal cord (SC) sections and their cross‐sectional area, the localization, percentage, and profile area of SPN containing neuronal nitric oxide synthase (nNOS) and calbindin (CB) in the thoracic SC of rats of different ages (from birth to 1‐year‐old) after capsaicin treatment. Neonatal capsaicin treatment generally decreased the cross‐sectional area of the SPN pericarya. However, the cross‐sectional area of the CB‐immunoreactive (IR) SPN increased in the central autonomic area in rats aged 10–30 days old after capsaicin treatment. The number of SPN decreased only in the central autonomic area of rats aged <20 days. The proportion of nNOS‐IR neurons remained steady and did not change during development. The cross‐sectional area of nNOS‐IR SPN in capsaicin‐treated rats was less than that in control rats. The results obtained will promote further studies on the mechanisms of sensory processing in the SC and the development of the sympathetic nervous system.