The subtype 1 neurotensin receptor (NTS1) belongs to the family of G protein coupled receptors with seven transmembrane domains and mediates most of the known effects of neurotensin. In the past years, mutagenesis studies have allowed to delineate functional regions of the receptor involved in agonist and antagonist binding, G protein coupling, sodium sensitivity of agonist binding, and agonist-induced receptor internalization. These data are reviewed and discussed in the present paper.