The effects of iontophoretically applied calcitonin gene-related peptide (CGRP) and/or substance P (SP) onto physiologically identified neurons in the rat lumbar dorsal horn were studied. The consistent excitatory effects of SP found on the spontaneous and the noxious evoked activity were significantly potentiated by CGRP application. This peptide facilitated the noxious evoked activity and did not affect the spontaneous activity of the neurons. The responses to non-noxious stimuli were unaffected in all cases.