The biological actions of somatostatin are mediated via a family of G protein-coupled receptors named sst1 to sst5. We used an affinity-purified polyclonal antibody AS-65, directed against a specific N-terminal peptide sequence of sst1 to determine the immunohistochemical distribution of N-terminal sst1 immunoreactivity in the rat brain. The specificity of the antibody was shown by western blotting experiments using an N-terminal sst1 fusion protein. Enzymatic deglycosylation experiments were combined with blotting experiments on a sst1-transfected cell line and rat brain membrane proteins and with immunocytochemistry on an sst1-transfected cell line. These studies showed that the antibody detected the deglycosylated sst1 receptor protein. Immunohistochemical staining showed that sst1 immunoreactivity (presumably the deglycosylated receptor) recognised by this N-terminal antiserum was widely distributed throughout the brain with cells and processes labelled in the cerebral cortex, regions of the limbic system (including the hippocampal formation and some basal ganglia nuclei), the epithalamus, the thalamus, different subthalamic structures (subthalamic nucleus, zona incerta), the colliculi, the hypothalamus, the reticular formation, the cerebellum and regions of the trigeminal nerve complex. The distribution of immunoreactivity was in good general agreement with that predicted from the localization of sst1 messenger RNA and radioligand binding studies. This study on the immunohistochemical distribution of the sst1 receptor in the brain will provide a better understanding of the central actions of somatostatin at its receptor types.