In heterothermic animals heart rate and the degree of autonomic tone on the heart typically varies with temperature. A predominant inhibitory tone is exerted by the parasympathetic system via the vagus nerve that is supplied by vagal preganglionic neurones (VPN) in the brainstem (Taylor et al 1999). This study measured autonomic tone on the heart of the bullfrog (Rana catesbeiana) at 3 temperatures and correlated the results with a neuranatomical study of the distribution of VPN in the brainstem. The results are compared with an earlier study on the toad, Xenopus laevis (Taylor and Ihmied 1995; Wang et al 1999). Bullfrogs (158 ± 30.9 g; n = 37) were kept at 25°C with free access to water. Food was withheld 3 days before experiments, performed in accordance with ethical guidance at UNESP. Frogs (n = 5) were anaesthetized in a solution of MS 222 (500 mg l-1). The cervical vagus was exposed for injection of 6-8 μL of a 2% solution of the neural tracer True Blue (Sigma). The animals were held at 20°C for 5 to 15 days then terminally anaesthetised in MS 222 and perfused with heparinised 9% saline then a 4% solution of formaldehyde buffered to pH 7.3. The brain was sectioned and fluorescing neurone cell bodies were counted and mapped according to their proximity to the 4th ventricle and their rostro-caudal distance from obex. VPN were located in a single nucleus within the central grey distributed over a rostro-caudal extent of 5mm and appeared to be of similar size. In contrast, Xenopus had a distinct lateral group of larger VPN located in the white matter outside of the central grey, constituting about 30% of total VPN. For measurement of heart rate frogs (n= 48) were anaesthetised in benzocaine (1 g L-1). The femoral artery was occlusively cannulated, with lidocaine applied to the incision. The frogs were held for 24 hours in climatic chambers at 10, 20 or 30°C. Blood pressures were recorded via a data acquisition system sampling at 100Hz. The β-adrenergic antagonist propranolol or the cholinergic muscarinic antagonist atropine were injected intra-arterially (3 mg kg-1 each), with the order of injection alternated and the efficacy of the autonomic blockade verified by injection of agonists. The adrenergic and cholinergic tones on the heart were calculated from cardiac intervals (1/fH). Data were evaluated using a paired t-test. Differences in heart rate were considered statistically significant at a 95% level of confidence (P < 0.05). Although heart rate increased with temperature, cardiac vagal tone in Bufo was low at all 3 temperatures, varying between -5% and +10% between 10 and 30°C. Studies on Xenopus revealed that vagal tone varied from 75% at 5°C to 185% at 15°C and 329% at 25°C. It seems possible that high levels of cardiac vagal tone are associated with dual origins for VPN in these amphibians.