The central effects of capsaicin, veratrine, histamine and bradykinin were studied by injecting them directly into the cerebrospinal fluid and their peripheral effects were examined by injecting into femoral vein. Our experiments were performed in Na-pentobarbital-anaesthetized dogs. Tidal volume (V-T), respiratory frequency (f/min), systemic arterial pressure (BP) were recorded. A significant increase in f, and an initial apnea or hypoventilation followed by a significant increase in V-T were observed with central and peripheral capsaicin. Vagotomy removed the peripheral V-T response, but not the central one. While central capsaicin administration increased BP, peripheral administration decreased. After vagotomy, a significant increase was observed in BP for both administrations. Respiratory responses to central and peripheral administrations of veratrine mere similar to those of capsaicin. Significant increases were observed in f and V-T of the intact group in response to central and peripheral administration of histamine. Response to peripheral administration disappeared after vagotomy. While central and peripheral bradykinin increased V-T significantly, there was no significant change in f. Vagotomy only removed the increase in V-T in response to peripheral administration. In conclusion, respiratory responses to central administration of capsaicin and veratrine are due to direct effects of these substances on respiratory neurons. In peripheral administration, disappearance of the responses after vagotomy indicate that the responses are brought about by stimulation of the lung receptors. (C) 1997 Tohoku University Medical Press.