The effects of intracerebral microinjections of substance P and cholecystokinin on self-stimulation of the medial prefrontal cortex of the rat were studied. Intracerebroventricular administration of substance P at doses of 2.5, 5, 10 and 20 micrograms produced a dose-related decrease in self-stimulation of the medial prefrontal cortex; spontaneous motor activity, measured as a control, was not affected. Unilateral microinjections into the medial prefrontal cortex of substance P at doses of 10 and 20 micrograms produced a decrease of self-stimulation of the ipsilateral side, but self-stimulation of the contralateral cortex, used as a control, was not affected. On the contrary, cholecystokinin in both intracerebroventricular administration at doses of 100, 200 and 400 ng, or intracortical microinjections into the medial prefrontal cortex at doses of 200, 400 and 800 ng, had no effect on self-stimulation of this cortical area. These results suggest that substance P, but not cholecystokinin, could be part of the neurochemical substrate underlying self-stimulation of the medial prefrontal cortex in the rat.