For the long-term continuous administration of drugs, a new osmotically powered infusion pump using semipermeable hollow fibres has been designed. Sufficient osmotic pressure, of about 980 mmHg, as a driving force the pump was obtained when hypertonic solution made of water-soluble polyethylene glycol (PEG) with 120 gl−1 was used. The prototype pump designed consist of: (i) a drug chamber; (ii) a chamber filled with PEG solution; (iii) a flow-control ring and clamps for flow-adjustment; (iv) a diaphragm bag filled with distilled water. In this pump eight values of the delivery rates ranging from 0 to 20 μl min−1 can be easily adjusted in 2·86 μl min−1 steps by choosing the combinations of three bundles of hollow fibres using the clamps. Fairly good pump charactersitics, including stability for long-term continuous infusion tested in vitro, were obtained. This pump seems to be practically useful for the infusion-therapy of diabetic and hypertensive patients and those with other diseases.