Concentrations of the principal electrolytes and respiratory gases were measured in serum of the land crab, Gecarcinus lateralis, and a physiological salt solution was prepared. The solution contains: sodium, 140 meq/l; potassium, 10 meq/l; magnesium, 14 meq/1; calcium, 18 meq/l; chloride, 462 meq/l; sulfate, 10 meq/l; "Tris" buffer, l0mmol/ 1; and is adjusted to pH 7. 2 with 0. 2 [image] maleic acid; it sustained axonal conduction, neuromuscular transmission, and muscular contraction for at least 4 hours in isolated nerve-muscle preparations from the animal. The serum osmolality was found to vary directly with the tonicity of the available water supply, indicating that Gecarcinus is capable of only limited osmoregulation.