Cholecystokinin octapeptide (CCK-8) applied microiontophoretically causes a moderate to strong excitation of about half of all tested dorsal horn neurons located in laminae I-VII of both the cat intact spinal cord and the rat in vitro spinal cord slice preparation. In the cat intact spinal cord the excitation is not limited to a single population of neurons but is observed in all categories of units recognized in spinal preparations of cats in this area on the basis of their excitability by different kinds of cutaneous afferent input. In the spinal cord slice preparation the excitatory action of CCK-8 persists even when the spinal cord slices are perfused with a Ca2+-free, Mg2+-high Krebs solution. The latter finding indicates that the action of CCK-8 might be a direct one exerted on the postsynaptic sites of dorsal horn units. These results are consistent with the possibility that CCK-8 acts on postsynaptic sites in the dorsal horn of the spinal cord as a neurotransmitter or modulator.