An insulin-like immunoreactivity (ILIR) was localized in the neuronal somata, dendrites and myelinated axons in the gracile nucleus of the male Wistar rat. In the neuronal somata, the reaction product was dispersed in the cell nucleus and cytoplasm. In the cell nucleus, the reaction product was scattered throughout the nucleoplasm, but not within the nucleolus. In the cytoplasm, the reaction product was evenly distributed, mainly in the vicinity of the cistemae of the rough endoplasmic reticulum. In labelled dendrites, the reaction product was closely associated with the parallel arrays of neurotubules and postsynaptic densities. Most of these labelled dendrites were postsynaptic to unlabelled axon terminals. A labelled dendrite often formed the central element of a synaptic glomerulus with several unlabelled axon terminals. Numerous labelled myelinated axons were also present in the neuropil. However, axon terminals appeared to be unlabelled. It is hypothesized that insulin-like substance(s) may be modulating nuclear activities as well as neurotransmission at the synapse in the gracile nucleus.