The neuropeptide Neuromedin U (NMU) stimulates smooth muscle contraction, and modulates local blood flow and adrenocortical function via two endogenous receptors, NMU1 and NMU2. Although its amino-acid sequence is highly conserved across species, the physiological effects of NMU are variable between species and little is known of its effects on human tissues. We have examined the contractile effects of NMU-25 on human smooth muscles of the gastrointestinal (GI) tract (ascending colon, gallbladder) and long saphenous vein (LSV) using in vitro organ bath bioassays. From LSV, ileum, gallbladder, caecum and colon, NMU receptor transcripts were amplified by RT-PCR and expression levels were determined by semi-quantitative scanning densitometry. NMU-25 produced a concentration-dependent, sustained contraction of isolated smooth muscle (p[A](50)+/-s.e.m., ascending colon, 8.93+/-0.18; gallbladder, 7.01+/-0.15; LSV, 8.67+/-0.09). NMU1 and NMU2 receptor transcription was detected in all tissues; transcription of both receptors was similar in gallbladder, but NMU1 receptor transcription was predominant in the sigmoid colon and LSV. In summary, these studies indicate that NMU may control tone in the human GI tract and LSV through an action on smooth muscle. Development of NMU receptor subtype-selective ligands will aid the further elucidation of the physiological roles of NMU and its two receptors.