The time-course of the fluxes causing net sodium uptake through the anal papillae, and the control of this uptake, have been studied in fed sodium-deficient fourth-instar larvae of the mosquito Aëdes aegypti in 2 mM/1 NaCl. Influx, outflux and uptake rate all decline exponentially with time, suggesting that effective negative feedback operates continually upon the transporting system. Uptake is impaired or eventually stopped when certain tissues are destroyed, or ligatured or clamped out of the

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... circulation. The effect is not explicable in terms of impaired circulation of haemolymph through the papillae, and is taken to indicate that a hormone is involved in sodium uptake. Interruption of the nerve cord suggests that an abdominal monitoring centre conveys information forwards along the nerve cord to a control centre in the thoracic ganglia to inhibit or increase hormone production. The hormone is apparently produced by the thoracic ganglia and the retro-cerebral complex (corpora allata and homologues of the prothoracic glands combined). Evidence that a second hormone, produced in the head, is also involved in sodium uptake is at present inconclusive. A possible mechanism of hormone action is suggested and discussed in relation to what is known of the functioning of the ionic carriers in the anal papillae.