5-Hydroxytryptamine (5-HT) and epinephrine were applied by microiontophoresis to single neurons in the isolated spinal cord of the frog. 5-HT depressed all but two of the responsive cells, whereas the response to epinephrine consisted exclusively of depression. 5-HT action was more marked than that of epinephrine on most cells. With either compound, responseve units were diffusely distributed throughout the tissue. While it was proven that prostaglandin E1 (PGE1) exerts a direct excitatory action on spinal neurons, no evidence of an antagonism between PGE1 and the monoamines was obtained. These findings provide additional support to the hypothesis that 5-HT and epinephrine are transmitters in the frog spinal cord. The possibility that PGE1 may 'modulate' the responsiveness of spinal neurons to the monoamines was not confirmed.