1. The heart rate of normal Xenopus laevis increased with acclimated temperature (5, 15 and 25°C) with a Q 1 0 of about 1.4.2. The injection of atropine abolishing a cholinergic vagal tone and raised heart rate. The proportional increase in heart rate (atropinized h.r./normal h.r.) increased with temperature so that the Q 1 0 increased to about 2.5.3. Injection of propranolol decreased heart rate, indicating that there was a β-adrenergic tone on the heart.4. Propranolol injection followed by atropine resulted in a heart rate higher than control but below the atropinised rate. This is likely to be the intrinsic heart rate, which also varied with temperature, having a Q 1 0 of about 2.5. Temperature related changes in heart rate are determined centrally and controlled chiefly by variations in cardiac vagal tone.