1. Sodium and lithium are actively transported by the isolated midgut of Hyalophora cecropia.
2. The short-circuit current in 32 mM sodium solution is about half of that in 32 mM potassium solution.
3. The sodium flux measured with 22Na from blood-side to lumen accounts for all of the short-circuit current and is 19 times the flux from lumen to blood-side.
4. In a solution containing 16 mM potassium and 16 mM sodium there is no transport of sodium, although a large current remains.
5. The sodium transport mechanism is not sensitive to ouabain.