The distribution of Na+, K+ and water was studied in spinal cord, sciatic nerve and sartorius muscle of the toad, Bufo arenarum (Hensel). Electrolyte assays and sodium washout curves were made. In sartorius muscle, extracellular water was estimated to account for 18.9% of total tissue weight and the intracellular concentrations of Na+ and K+ were 18.4 and 147.9 mEq/kg of intracellular water respectively. In spinal cord extracellular water was 17.2%, intracellular Na+ was 40.6 mEq/kg of water, and intracellular K+ was at a concentration of 134.5 mEq/kg. Interpretation of washout curves in sciatic nerve could not be as simple as in other tissues due to the connective sheath, and therefore electron micrograph measurements had to be made in order to estimate how much of the total tissue weight could be attributed to the sheath. Assuming that it had a water and electrolyte composition similar to that of plasma, the sheath corresponded to 22.3% of the nerves and contained a proportional fraction of Na+ and K+. These calculations left an 'excess' of Na+ and water to be distributed among the remaining components of sciatic nerve. Application of the pyroantimoniate histochemical technique for Na+ determination disclosed a large amount of precipitate among myelin lamellae both in sciatic nerve and in spinal cord. This might explain at least in part the distribution of the 'excess' Na+ in both nervous tissues.