In critical ill patients, determination of blood gases is a crucial part of patient evaluation and management. In intensive care situations many blood-gas determinations are required in a single day. The introduction of an arterial catheter into a small peripheral artery is an invaluable technique that allows continuous blood-sampling. During the past three years we had catheterized either the radial, ulnar or the pedal artery in 450 instances. The arterial catheter was left in place from 1 to 15 days. There were no serious complication. Haemorrhage due to accidental opening of the catheter occurred in three occasions. No case of ischaemia was encountered after a single artery catheterization. Temporary ischaemia of the upper limb appeared in three patients after simultaneous catheterization of both ulnar and radial arteries. The infection rate was nil. Arterial catheterization and maintenance of indwelling catheter for repeated blood-samples is a simple and safe procedure with minimal complications, invaluable in the management of critical ill patients.