The present study describes the structure and development of the flame-shaped central arborizations of hair follicle (HF) afferents in the lumbosacral spinal cord of the rat. Tissue was processed according to the rapid Golgi method at successive stages of development from embryonic day 17 through postnatal day 30. Collaterals of most HF afferents were found to enter the dorsal horn via a characteristic U-shaped pathway which often parallels the vascular pattern. The HF collaterals can first be identified at embryonic day 19 and by postnatal day 5 they have established the dorso-ventral and rostro-caudal limits of their field of arborization. Dorsally the arbors extend no further than the inner zone of lamina II (IIi) at any stage of development. Short aberrant branches were observed on some HF collaterals during the prenatal period but none of these developed synaptic terminals or contributed branches to other arbors. Each HF collateral formed a single well defined flame-shaped arbor with a distribution and branching pattern which could be distinguished from that of other afferents throughout the postnatal developmental period. Two types of HF collaterals were observed. Structure and distribution patterns suggest that type I collaterals are derived from G and T HF afferents while collaterals in the type II category are probably derived from both G and T as well as D (A-delta) HF afferents. Type I collaterals divide into well defined medial and lateral collateral branches which arborize mainly in lamina III with a few branches to lamina IV. Type II collaterals are characterized by a delicate arbor which is more vertically oriented than type I arbors.(ABSTRACT TRUNCATED AT 250 WORDS)