An adjustable pump for microfluidics employing principles of osmoregulation analogous to those of phloem loading in plant leaves has been constructed and tested. Volume flow arises in a hollow fibre with vapour-permeable hydrophobic membrane. The fibre is connected to a source chamber filled with salt crystals and saturated salt solution. The source chamber takes up water through a relatively small membrane area and delivers saturated salt solution to one end of the capillary flow path within the hollow fibre. A stationary osmotic gradient is sustained in the hollow fibre lumen by constant input of saturated salt solution and radial osmotic water absorption. The strong temperature dependence of isothermal membrane distillation enables adjustment of the flow rate up to 20 nL/s. The pump provides pulse-free flow of any liquid with constant rate for at least 26 days without recharging the source chamber. Backpressures up to 1 bar decrease the flow rate by less than 4%. The volume delivered at a constant rate is more than 40 times larger than the volume of the source chamber. Osmoregulatory pumps of the described type may be useful for microinfusion, microdialysis and analytical microsystems. (c) 2006 Wiley Periodicals, Inc.