1. The excretion of calcium, magnesium, chloride, phosphate and hydrogen ions has been studied in preparations of single Malpighian tubules isolated in drops of serum under liquid paraffin.
2. The concentrations of calcium, magnesium and chloride are always lower in the urine than in the serum.
3. The concentration of phosphate is always greater in the urine than in the serum. As the concentration of phosphate in the serum increases, the rate of urine flow also increases.
4. The urine is always alkaline to the serum but becomes acid in the rectum.
5. The general problem of excretion by Malpighian tubules is briefly reviewed and discussed.