Malpighian tubules of insects typically secrete an iso-osmotic fluid by a process which is thought to involve the following: (1) Potassium ions enter the tubule cells by a process which is sodium dependent and which may be active; they are then actively pumped into the lumen by an electrogenic pump. (2) Sodium ions cross the wall in a similar fashion to potassium ions but their entry into the cells is very restricted so that they are transported only slowly. (3) These active cation movements create a trans-wall potential favouring the passive movements of anions from the haemolymph into the lumen. (4) With one exception, smaller anions appear to cross the wall faster than do larger ones. The exception is that phosphate ions cross faster than any other anion in spite of their large size. The evidence suggests that this is more likely to be achieved by facilitated diffusion but active movements are not excluded. (5) The apical and basal membranes of the tubule cells are elaborately folded. It is suggested that these foldings act to couple movement of water to the movements of ions by allowing the development of standing osmotic gradients. (6) Such gradients will be small because the channels in which they occur are short. However, as the cell membranes probably have a high osmotic permeability water is likely to be able osmotically to equilibrate with the channel contents to produce an iso-somotic secretion. (7) The folds in the cell membrane are such that a parallel array of channels alternately opening to the cytoplasm and to the extracellular fluid is produced. Such an arrangement leads to a steeper osmotic gradient across the cell wall and this will promote a more efficient coupling of solute and water movements.