Immunofluorescence was used in the rat to study the early ontogenetic expression of somatostatin (SRIF) in the dorsal root ganglia (DRGs) from gestational day 10.5 to day 15.5. SRIF-immunoreactivity (IR) was not detectable in day-10.5 embryos, was first observed in DRGs at day 11.5, reached a peak in intensity and distribution at around day 13.5 and thereafter decreased to become undetectable by day 15.5 in the DRGs of the trunk region. The dynamic expression of SRIF-IR in DRG perikarya could be correlated with its expression in nerve fibers located in the limbs and the abdominal mesenchyme. Thus, SRIF-IR is expressed at a time when sensory fibers could have established connections with their embryonic targets and when DRG neurons could have undergone their final mitotic phase. These data showing the earliest and transient expression of a neuropeptide in developing DRGs confirm and extend the notion that SRIF plays an important role in developmental processes.