The muscarinic receptor present on the guinea-pig uterus has been characterized by use of both functional and radioligand binding studies. The contractile responses to carbachol were antagonized by atropine, pirenzepine, methoctramine and hexamethonium with pA2 values consistent with the presence of an M2 receptor (i.e. 9.5, 7.0, 8.0 and 3.7 respectively). In binding studies, pKi values were also observed for pirenzepine, methoctramine and AF-DX 116 which were consistent with the presence of a homogeneous M2 receptor population (i.e. 6.6, 7.9 and 7.1 respectively). The uterus therefore possesses M2 receptors through which a functional contractile response is mediated and in this respect it is clearly different from other smooth muscles. M2 receptors in the uterus mediate excitatory responses, an effect in contrast to their action in other tissues.