The ionic dependencies of stimulated and unstimulated Locusta tubules have been studied. K+, Na+, Cl- are essential to both basal and stimulated secretion. K+ is secreted against a concentration gradient in unstimulated tubules. In response to diuretic hormone or cAMP application, there is a dramatic influx of K+ into the lumen. A high level of Na+ and Cl- in the bathing medium is required to allow maximal fluid secretion. The tubules show an apparent impermeability to Na+; its concentration in the secreted fluid is always much less than in the bathing medium. If Na+ is omitted from the medium and excess K+ added (80 mM K), then although basal secretion occurs (2.5 nl/min), the tubules fail to respond to stimulation. Clearly Na+ has an important indirect role to play in stimulated fluid secretion.