Calcitonin gene-related peptide (CGRP) and substance P (SP) play an important role in the development of pain and hyperalgesia. Experimental models have demonstrated that nitroglycerin (NTG)--a nitric oxide donor--provokes a hyperalgesic state, probably via the activation of second-order neurons in the nucleus trigeminalis caudalis. In order to gain further insight into the role of CGRP and SP in different types of experimental pain, we evaluated and compared changes in immunoreactivity (-ir) for these two neuropeptides at different levels of the central nervous system [nucleus trigeminalis caudalis (NTC) and dorsal horns of the lumbar spinal cord] in two animal models of hyperalgesia: systemic NTG administration and formalin test. Following NTG administration, CGRP-ir decreased steadily in the NTC, whereas SP-ir increased transiently. In the lumbar dorsal horns, NTG induced a decrease in SP-ir 1 h after its administration. Formalin injection induced an ipsilateral increase in both CGRP and SP immunostaining at 1 and 2 h in the lumbar dorsal horns. In the NTC, a significant decrease in CGRP-ir was observed at 1 h. The changes in the staining intensities were paralleled by changes in the numbers of CGRP and of SP varicosities in both the NTC and the lumbar dorsal horns. These findings show specific changes in CGRP and SP at different levels of the central nervous system in the different models of pain. In the case of the formalin test, the changes involve both neuropeptides synchronously and to the same extent, whereas in the case of NTG administration, CGRP seems to play a more prevalent and long-lasting role, particularly at the NTC level.