Nociceptin/orphanin FQ (N/OFQ) is the endogenous ligand for the N/OFQ peptide receptor (NOP). N/OFQ causes hypotension and vasodilation and we aimed to determine the role of histamine in inflammatory microvascular responses to N/OFQ. Male Wistar rats (220-300g, n=72) were anesthetised with thiopental (30mg.kg(-1) bolus, 40-90 mg.kg(-1)hr(-1) intravenous; i.v.) and the mesentery prepared for fluorescent intravital microscopy (IVM) using fluoroscein isothiocyanate conjugated bovine serum albumin (FITC-BSA) (0.25ml.1(-1)00g-1, i.v.) or 1microm fluorescently labelled microspheres. N/OFQ (0.6-60nmol.kg-1, i.v.) caused hypotension (SAP, baseline: 154 +/-11mmHg, 15nmol.kg N/OFQ: 112 +10mmHg, P=0.009), vasodilation (venules: 23.9 +/-1.2microm, 26.7 +/-1.2microm, P=0.006), macromolecular leak (interstitial gray level FITC-BSA: 103.7 +/-3.4, 123.5 +/-11.8, P=0.009) and leukocyte adhesion (2.0 +/-0.9, 15.2 +/-0.9 per 100microm, P=0.036). Microsphere velocity also decreased (venules: 1230 +/-370mm.sec-1, P=0.037), but there were no significant changes in blood flow. Flow cytometry measured a concurrent increase in neutrophil expression of cd11b with N/OFQ versus controls (Geo Mean Fluorescence: 4.19 +/-0.13 versus 2.06 +/-0.38, P<0.05). The NOP antagonist [Nphe1,Arg14,Lys15]N/OFQ-NH2 (UFP-101; 60 and 150 nmol.kg(-1) i.v.), H1 and H2 antagonists pyrilamine (mepyramine, 1mg.kg(-1) i.v.) and ranitidine (1mg.kg(-1) i.v.) and mast cell stabiliser cromolyn (1mg.kg(-1)min(-1)) also abolished vasodilation and macromolecular leak to N/OFQ in vivo (P<0.05), but did not affect hypotension. Isolated mesenteric arteries (~200microm, n=25) preconstricted with U46619 were also mounted on a pressure myograph (60mmHg) and both intraluminally and extraluminally administered N/OFQ (10(-5)M) caused dilation; inhibited by pyrilamine in the extraluminal but not the intraluminal (control: -6.9 +/-3.8%, N/OFQ: 32.6 +/-8.4%, pyrilamine: 31.5 +/-6.8%, n=18, P<0.05) experiments. We conclude that, in vivo, mesenteric microvascular dilation and macromolecular leak occur via N/OFQ-NOP mediated release of histamine from mast cells. N/OFQ-NOP therefore, has an important role in microvascular inflammation and this may be targeted during disease, particularly as we have proven that UFP101 is an effective antagonist of microvascular responses in vivo. Key words: nociceptin/orphanin FQ, microcirculation, vasodilatation, inflammation, NOP.