SUMMARY

1. Sodium occurs at a much lower electrochemical potential in all gut regions than in the blood. The observed gradients do not decrease with time; the maintenance of the high blood sodium levels therefore requires active uptake against an electrochemical gradient. 2. Potassium occurs at a much higher electrochemical potential in all gut regions than in the blood. The observed gradients do not decrease with time; the maintenance of the low blood potassium therefore requires active excre-tion against an electrochemical gradient. The highest luminal concentration is observed in the posterior midgut. 3. Chloride appears to be passively distributed across the gut. As with potassium, the highest luminal activity is observed in the posterior midgut. 4. Calcium and magnesium are similarly distributed. They both occur at a fairly high level in the blood, crop, caeca and anterior midgut, and at a much lower level in the rest of the gut. 5. The crop and caecal fluids become markedly hyperosmotic to the blood in freshly-fed locusts, but the midgut fluid is isosmotic at all times. The fluid in the ileum is significantly hyposmotic; the shortfall in osmolality, compared with the fluid in the posterior midgut, corresponds to the observed drop in KC1 level. 6. The anterior caeca are the only parts of the alimentary canal to possess a lumen-negative transepithelial potential difference. They differ significantly from the other regions studied, notably the posterior caeca, in this respect