In a prospective randomized controlled trial involving 470 arterial catheters in 470 children, we studied the effect of changing either the concentration or the flow rate of a heparin infusion. Although catheters tended to remain patent longer with a flow rate of 2 ml/h rather than 1 ml/h, the difference was not statistically significant. Increasing the heparin concentration from 1 to 5 U/ml significantly prolonged catheter patency.