The aim of this study was to examine the growth of fine-diameter, A delta and C, primary afferents into the dorsal horn of the spinal cord. To do this, A delta and C primary afferents were transganglionically labeled with wheat germ agglutinin-horseradish peroxidase (WGA-HRP) in fetal rats at various ages and their growth was traced into the lumbar spinal cord. Sciatic or common peroneal nerves were injected with WGA-HRP at E18-E21.5 (just before birth) and the fetuses were returned to the uterus for 24 h before histochemical analysis. The results show fine-diameter afferent fibers growing into the L4/L5 spinal cord at E19 when they reach the white matter overlying the dorsal horn and begin to penetrate lamina I. Twelve hours later at E19.5 terminals can be seen in lamina IIo and by E20 they are increasing in density and have reached IIi. By birth they have achieved the density in laminae I and II that is found in the neonate and young rat. Labeling of only the peroneal branch of the sciatic nerve, leaving the tibial unlabeled, demonstrated that the somatotopic specificity of central terminal fields is apparent from the time fine afferent fibers first grow into the spinal cord.