Electrophysiological, biochemical, and behavioural studies have suggested that opiate withdrawal is mediated, at least in part, by a hyperactivity of locus coeruleus (LC) neurones. The aim of this study was to evaluate, using single-unit extracellular recordings, the role of NO in the opiate withdrawal-induced hyperactivity of LC neurones in anaesthetized rats. In animals chronically treated with morphine (5 days), administration of naloxone caused an increase in the spontaneous firing rate of LC cells. Acute pretreatment with the nitric oxide synthase (NOS) inhibitor NG-nitro-L-arginine methyl ester (30 mg kg(-1) i.p.) attenuated some signs of opiate withdrawal (total score reduced by 55%), and also the withdrawal-induced hyperactivity of LC neurones (hyperactivity reduced by approximately 50%). Acute pretreatment with 7-nitro indazole (50 mg kg(-1) i.p.), a selective inhibitor of neuronal NOS, caused a complete blockade of the withdrawal-induced hyperactivity of LC neurones. Application of 7-nitro indazole (30 microM) in the vicinity of the LC also caused a reduction (of approximately 60%) in the withdrawal-induced hyperactivity of LC cells. Intravenous administration of these NOS inhibitors (after naloxone challenge) did not produce comparable changes in the LC cell firing activity. 7-Nitro indazole failed to affect the development of tolerance of the LC to the morphine effect in opiate-dependent rats (i.e. morphine dose-effect curves were shifted to the right by morphine treatments to a similar degree in vehicle- and 7-nitro indazole-pretreated rats). The present data suggest that opiate withdrawal might be mediated by nitric oxide acting as an intermediate messenger in the LC.