1. 1. Plasma potassium, calcium, and sodium were not appreciably changed with the injection of sodium arsenite and sodium arsenate in cats. 2. 2. Intravenous sodium arsenite was found to be about ten times more lethal than sodium arsenate, but arsenite has a little over twice as much arsenic content. Both compounds markedly lowered the blood pressure in large doses. 3. 3. Alternate injections of the arsenical solutions with histamine failed to change the marked plasma potassium rise consistently seen with histamine. 4. 4. With 90% of the liver removed no plasma cation changes were noted with saline or sodium arsenite. Histamine produced a rise in the potassium, but it was reduced to about one-third the normal magnitude. 5. 5. A transient pressor response was seen with sodium arsenate, and during this period the cardiac output was markedly increased. With sodium arsenite the hypotension was accompanied by a small decrease in cardiac output. Only when the hypotensive phase (30–60 mm Hg) persisted were the cardiac output decrease, the hematocrit, and circulation time markedly altered. 6. 6. In the rabbit skin test sodium arsenite caused a higher degree of capillary permeability change than sodium arsenate, but, as noted above, it has over twice as much arsenic content. 7. 7. Five to ten times the microgram dose of either of the arsenical compounds was needed to equal the capillary permeability changes produced by histamine in the rabbit skin. 8. 8. Arsenic changed the capillary permeability of the gastrointestinal tract, liver, and kidneys to a much greater extent than histamine. 9. 9. Capillary permeability changes and vascular smooth muscle effects of histamine per se did not seem to be instrumental in the production of hypokalemia.