Nociceptin/orphanin FQ (N/OFQ) is the endogenous ligand for the N/OFQ peptide receptor (NOP) and has been shown previously to produce bradycardia and hypotension in rodents. In this study we have measured the effects of intravenous N/OFQ, and the NOP antagonists [Nphe(1)]N/OFQ(1-13)-NH(2) ([Nphe(1)]) and [Nphe(1),Arg(14),Lys(15)]N/OFQ-NH(2) (UFP-101) on cardiovascular parameters and plasma catecholamine concentrations. Female Hartley guinea pigs were anesthetized with pentobarbital and ventilated artificially. MAP and HR were measured via a femoral arterial catheter and ECG, respectively. Plasma catecholamine concentrations were measured by HPLC. Animals received saline, N/OFQ (0.25, 1.25, 6.25 and 25 nmol cumulatively at 10-min intervals), [Nphe(1)] (600 nmol) and UFP-101 (60 nmol) i.v. in various combinations. After establishing a stable baseline, MAP and HR measurements and blood sampling were performed at the beginning and 3 min following each drug administration. N/OFQ significantly decreased MAP, HR and the plasma noradrenaline concentrations in a dose dependent manner (maximally by 29.1+/-1.8%, 13.8+/-0.8% and 46.6+/-7.8%, respectively) To the contrary, N/OFQ tended to increase plasma adrenaline concentration but did not affect plasma dopamine concentrations. There was a significant correlation between percent change in MAP (0.69, P<0.01) or HR (0.84, P<0.01) and that in plasma noradrenaline. [Nphe(1)], but not UFP-101, alone significantly decreased MAP. [Nphe(1)] partially antagonized N/OFQ-induced hypotension, bradycardia and the decrease in plasma concentration of noradrenaline. UFP-101 fully prevented the effects of N/OFQ in this model. In conclusion, the present study shows that intravenous N/OFQ, via NOP receptors, elicits hypotension and bradycardia also in the anaesthetized guinea pig and that the decrease in MAP and HR are positively correlated with the decrease in the plasma noradrenaline level.