With the indirect immunofluorescence technique of Coons and collaborators the occurrence of substance P (SP)-like immunoreactivity was studied in spinal ganglia (L6-S1), the spinal cord (L6-S1) and the pad skin of the hind paw of the cat. In untreated cats a very dense network of SP-positive fibers was found in the spinal cord in Lissauer's fasciculus, in laminae I-III and a rather dense plexus was seen in the ventral horns, in the area around the central canal (laminae X) and in the medial parts of laminae VI and VII. SP-positive fibers were also observed in the connective tissue under the epithelium of the skin. However, in untreated cats no specific immunogluorescnece was observed in the spinal ganglia, dorsal roots or certain large peripheral nerve trunks. After certain experimental procedures such as local application of colchicine or compression of the dorsal root close to the spinal ganglion, SP-positive fluorescence was observed in a rather small number of neuronal cell bodies and in fibers. The fluorescent material was observed in the peripheral parts of the cytoplasm and the cell bodies were exclusively of the small type. Ten days after transection of the dorsal roots a marked decrease in the number of SP-positive fibers was observed in the substantia gelatinosa but not in the ventral horns. The present results give strong evidence for the occurrence of SP in a certain population of primary sensory neurons and support earlier findings that SP may act as a transmitter or modulator in these neurons.