The localization of substance P (SP) or a SP-like peptide in cultured ganglia from chick embryos was studied by the indirect immunofluorescence technique. Ganglia from 8-16 days old chick embryos and from newly hatched chickens were cultured in a control medium or in the presence of nerve growth factor (NGF). Addition of colchicine and exposure to different explanted peripheral tissues were also tried. Ganglia from the younger embryos (8-12 days) cultured for 24 h with added NGF showed a weak SP-like immunoreactivity (SPLI) in some cell bodies and strong specific immunofluorescence in nerve fibres growing out from the ganglia. In spinal ganglia of the older embryos (14 and 16 days) and newly hatched chickens cultured with and without NGF the concentration of SPLI in the cell bodies was considerably higher. Addition of colchicine to spinal ganglia cultured 12 h in NGF-medium, resulted in retraction of nerve fibres and strongly fluorescent, expanded nerve fibres were observed in peripheral parts of the ganglia. Explants of skin placed near the spinal ganglia stimulated the outgrowth of fibres, some of them containing SPLI. A few fluorescent fibres were also seen within the skin explants. Also heart tissue explants stimulated outgrowth of nerve fibres, but innervation of these explants with SPLI-containing nerves could not be observed. Nerve fibre-extension from the spinal ganglia was not stimulated by spinal cord explants. The present results support the existence of SP-containing primary sensory neurons in chickens.