The localization and regulation of neuronal nitric oxide synthase (nNOS), inducible nitric oxide synthase (iNOS) and soluble guanylyl cyclase (sGC) were assessed in the spinal cord of mice stimulated by an intraplantar injection of zymosan. Both, nNOS and iNOS were upregulated in the dorsal horns of the spinal cord in response to the zymosan challenge. While nNOS was found in neurons of superficial laminae iNOS occurred in astrocytes. Thus, astrocytes might be involved in the processing of nociceptive stimuli. Expression of sGC was not affected by zymosan treatment. It was found exclusively in nerve fibers suggesting that it was predominantly localized to the presynaptic neuron. This supports the hypothesis that nitric oxide (NO) acts as retrograde messenger in spinal nociceptive processing.