Abstract 1. 1. Submersion causes bradycardia, during November-December, in Rana pipiens, R. temporaria, Bufo bufo and Xenopus laevis. Heart rate stabilizes after 30 min submersion. The underwater stabilized rate is similar in all species except X. laevis 2. 2. Adjustment from surface to underwater stabilized heart rate occurs most rapidly in R. pipiens. 3. 3. Rate of adjustment is slower and underwater equilibrium heart rate higher in summer R. temporaria and B. bufo as compared with winter and breeding animals. 4. 4. During hibernation of R. temporaria heart rate is low and similar in animals of a wide size range. 5. 5. Both seasonal and specific differences are discussed in terms of variations in nervous and metabolic activity.