In this combined electrophysiological-ultrastructural study in the cat spinal cord, we detected enkephalin-like immunoreactivity using internally radio-labelled monoclonal antibodies in functionally characterized neurons which had been filled intracellularly with horesradish peroxidase. Of the 4 neurons included in this study, two were positive for enkephalin immunoreactivity; one was a nociceptive specific neuron in lamina I, the other a wide dynamic range neuron in lamina V. The other two cells were devoid of immunoreactivity for enkephalin; one was a wide dynamic range neuron and the other was a nonnociceptive neuron. These results thus provide a morphological substrate within the spinal dorsal horn for the release of an endogenous opioid following administration of substance P or noxious cutaneous stimulation.