1. In the collembolan Podura aquatica L., uptake of water occurs both via the mouth and across the ventral tube vesicles.
2. Drinking is important in water-stress situations and when the haemolymph pressure is too low for eversion of the vesicles. Everted vesicles alone are sufficient for recovery from partial desiccation.
3. Experiments in which insects with their mouths or ventral tubes blocked were exposed to a medium containing 24Na indicated that most of the exchange of sodium between insect and medium occurred over the ventral tube vesicles, and that this exchange was rapid.
4. In experiments using 42K, no measurable exchange occurred between the insect and the medium.