1. The primary aim of this investigation was to determine whether binding sites corresponding to the 5-HT7 receptor could be detected in smooth muscle of the rat jejunum. Binding studies in rat brain (whole brain minus cerebellum) and guinea-pig ileal longitudinal muscle were also undertaken in order to compare the binding characteristics of these tissues. Studies were performed using [3H]-mesulergine, as it has a high affinity for 5-HT7 receptors. 2. In the rat brain and guinea-pig ileum, pKD values for [3H]-mesulergine of 8.0 +/- 0.04 and 7.9 +/- 0.11 (n = 3) and Bmax values of 9.9 +/- 0.3 and 21.5 +/- 4.9 fmol mg(-1) protein were obtained respectively, but no binding was detected in the rat jejunum. [3H]-mesulergine binding in the rat brain and guinea-pig ileum was displaced with the agonists 5-carboxamidotryptamine (5-CT) > 5-hydroxytryptamine (5-HT) > or = 5-methoxytryptamine (5-MeOT) > sumatriptan and the antagonists risperidone > or = LSD > or = metergoline > ritanserin > > pindolol. 3. Despite the lack of [3H]-mesulergine binding in the rat jejunum, functional studies undertaken revealed a biphasic contractile response to 5-HT which was partly blocked by ondansetron (1 microM). The residual response was present in over 50% of tissues studied and was found to be inhibited by risperidone > LSD > metergoline > mesulergine = ritanserin > pindolol, but was unaffected by RS 102221 (3 microM), cinanserin (30 nM), yohimbine (0.1 microM) and GR 113808 (1 microM). In addition, the agonist order of potency was 5-CT > 5-HT > 5-MeOT > sumatriptan. 4. In conclusion, binding studies performed with [3H]-mesulergine were able to detect 5-HT7 sites in rat brain and guinea-pig ileum, but not in rat jejunum, where a functional 5-HT7-like receptor was present.