1. The absorption of Na, K and Cl from the rectum has been studied in locusts previously supplied either with tap water or with hypertonic saline, the latter treatment resulting in a 40-70% increase in the ionic concentrations in the haemolymph.

2. Both water-fed and saline-fed locusts can absorb Na, K and Cl from the lumen of the rectum against concentration differences of up to 100-fold. The lumen is 15-30 mV. positive to the haemolymph. Absorption of Cl is certainly an active process; absorption of Na and K probably involves active processes.

3. In water-fed locusts absorption of K is four to twelve times more rapid than that of Na, and absorption of Cl is three times faster from KC1 than from NaCl.

4. In saline-fed locusts the relative rates of absorption of Na, K and Cl are the same as in water-fed locusts, except when the concentrations in the rectal fluid exceed those in the haemolymph.

5. The experimental results are consistent with the hypothesis that regulation of absorption of water and of ions from the rectum is brought about by changes in the passive permeability of the epithelium rather than by changes in mechanisms of active transport.

This study was carried out under tenure of a scholarship from the National Research Council of Canada.