1. The influences of asphyxia, temperature, and restraint on diving bradycardia in amphibia have been considered.
2. In restrained diving frogs two forms of bradycardia were distinguised on the bases of rate of attainment, control, and degree of hear-rate reduction. A rapidly attained immersion bradycardia could be eliminated by atropine or bilateral vagotomy and was thus similar to that of other vertebrates. A slowly attained asphyxiant bradycardia, independent of vagal influences, also occured but was less pronounced.
3. Electrical stimulation experiments were conducted in September to determine the influence of temperature on vagale inhibition of the heart. With decreasing temperature partial block of vagal inhibition occurred at about 11° C., and complete block occured at 7-8°C.; on rewarming, partial block returned around 11-12°C., and complete vagal inhibition of the heart was re-established at about 14° C.
4. Though both forms of bradycardia may occur in any one dive, the form predominating was dependent on temerature and season. A Seasonal variability regarding the temerature at which vagal inhibition of the heart can occur has been suggested.
5. A. pronounced diving bradycardia, either slowly or rapidly attained, probably does not occur in unrestrained frogs under normal environmental conditions.