The sartorius muscles of 320 toads have been analyzed for Na+ and K+. There is a wide variation in the Na+ content which when calculated intracellularly varied from 0 m.eq./kg. to 58 m.eq./kg. In particular it was found that the distribution of internal Na+ in the intact animal was such that only 17 per cent of the muscles should give from the Nernst equation the observed overshoot of 37 mv.
In contrast to this wide variation the K+ content is comparatively constant, the range being 71 to 112 m.eq./kg. The mean observed resting potential of 87 mv. agreed well with the potential calculated from the mean intracellular K+ by the Nernst equation.
Analyses of plasma show that the Na+ content is constant at 130 m.eq./liter, and the K+ is 3.0 m.eq./liter.
The resting and action potentials of 77 muscles have been recorded and then the muscles have been analyzed. The results have shown that there is no correlation between the level of intracellular Na+ and the overshoot. Furthermore the apparent correlation between the average K+ content and the average resting potential has been shown to be fortuitous, when the correlation in individual muscles is considered.
When a muscle is soaked in Ringer solution for several hours there is a gain of Na+ and a loss of K+. These shifts should result in changes in the respective potentials, but such changes were not found.
The above findings have been discussed in the light of the present theories that the resting potential and the action potential are directly related to the ionic ratio across the membrane. Our results very definitely do not support the theory that the overshoot is related to the Na+ gradient, and this also applies with respect to the K+ gradient and the resting potential.