1. The part played by the Malpighian tubules in the salt and water balance of Aedes aegypti larvae has been studied; the intestinal fluid and haemolymph have been compared in respect of freezing-point depression and sodium concentration.
2. It appears highly probable that the fluid passing down the intestine is derived from the Malpighian tubules with little or no contribution from the midgut.
3. When the larvae are kept in fresh water the intestinal fluid is very slightly hypotonic to the haemolymph (not isotonic as previously reported), but its sodium concentration is only about one-half that of the haemolymph.
4. When the larvae are kept in solutions of NaCl the difference in sodium concentration between intestinal fluid and haemolymph decreases. In an external medium of 1% NaCl the difference is abolished.
5. There is thus evidence that when the external medium is poor in salts the Malpighian tubules can contribute to the work of salt retention by excreting a fluid containing less sodium than the haemolymph; but there is no evidence that under any conditions they can excrete a fluid containing more sodium than the haemolymph.
6. Evidence of a decrease in the sodium concentration of the tubule fluid from distal region to proximal region is not statistically significant.