Previous studies, using non-specific nitric oxide synthase (NOS) inhibitors, have shown that nitric oxide (NO) has a significant facilitatory effect on the actions of the vagus nerve on several aspects of cardiac function. The present study aims to identify a potential neuronal site for the action of NO by using the n-NOS inhibitor, 1-(2-trifluoromethylphenyl) imidazole (TRIM) in the ferret and other mammals. The effects of TRIM on vagally evoked alterations in heart rate and atrio-ventricular (a-v) conduction in the anaesthetised ferret, rabbit and guinea pig are described. In ferrets with both vagi sectioned and repeated infusions of propranolol, the vagally evoked, frequency-dependent bradycardia was significantly attenuated by infusion of TRIM (10-30 mg kg(-1)). This effect was reversed by subsequent infusion of L-arginine (20-6 mg kg(-1)). TRIM also attenuated to a similar extent the vagally evoked bradycardia in similarly prepared guinea pigs, but NOS inhibition and the use of the NO donor, molsidimine, failed to alter the heart rate effects of vagal stimulation in the rabbit. In studies on a-v conduction (dromotropy) in the ferret, electrical stimulation of the left cervical vagus increased the a-v conduction time in a frequency-dependent manner. Administration of TRIM (30 mg kg(-1)) significantly attenuated this response. Again, L-arginine (60 mg kg(-1)) reversed it. Since an alteration in heart rate may have a concomitant action on a-v conduction time, the effects of vagal stimulation on a-v conduction were also carried out in ferrets with the heart paced at a constant rate electrically. There was no significant difference between the effects of vagal stimulation obtained from hearts which were paced and those which were unpaced. This implies that vagal stimulation had a direct effect on a-v delay and the changes were not secondary to alterations in cardiac rate. Based on other evidence that TRIM is a powerful reversible n-NOS inhibitor in vivo, our studies support strongly the hypothesis that NO liberated from neuronal sources has an important facilitatory action on the vagal control of the heart. In relation to vagal heart rate control, it has now been shown that, in line with other studies in the dog and the rat, NO exerts a powerful facilitatory action in the ferret and the guinea pig but not in the rabbit. It is to be expected that these effects of NO will also be demonstrable on other vagal cardiac actions.