BACKGROUND The spinal trigeminal nucleus and the dorsal horn of the spinal cord are the critical areas in relaying noxious impulses. They also contain large amounts of somatostatin-like immunoreactivities. Early publications focussed on immunohistochemical studies and were devoid of a detailed description of the distribution of somatostatin in the two nuclei at the molecular level. METHODS Frontal tissue sections from the mudulla and the spinal cord of eight rats were examined and a non-radioactive in situ hybridization histochemical procedure was adopted to study the distribution of somatostatin mRNA positive neurons in the two nuclei. RESULTS A widespread distribution of somatostatin mRNA containing neurons was shown in the two nuclei at all levels. The positive neuron profiles were normally round or oval in shape and small to medium in size. Three types of cells were identified, which were associated with the intensity of the hybridization signals. The highest density of somatostatin mRNA positive neuron profiles was found in the gelatinous subnucleus at the caudal part of the spinal trigeminal nucleus and in the substantia gelatinosa of the dorsal horn at the levels of the cervical and lumbar cords. Most of them belonged to type I neurons. CONCLUSIONS The present findings are compared to reports of previous studies. It is suggested that somatostatin in the two nuclei may play an important role in the modulation and transmission of pain signals.