1. Individual limbs of spinal amphibians when passively retracted respond by active protraction.
2. When both forelimbs or both hindlimbs are simultaneously subjected to passive retraction they respond alternately and not simultaneously.
3. When all four limbs are simultaneously subjected to passive retraction their order of response follows the diagonal pattern typical of normal ambulation. When all four limbs are in contact with a moving platform, the diagonal rhythm of limb protraction is sustained for long periods.
4. In the intact toad or frog, a protractor reflex in one limb is accompanied by retraction in the ipsilateral fellow limb and by protraction of the diagonal limb.
5. The normal ambulatory cycle in amphibia is to be regarded as a series of co-ordinated reflexes, largely dependent upon proprioceptor sense organs, and not as the expression of a centrally determined rhythm.