PURPOSE: The purpose of this study was to compare saline and heparin flushes in 24-gauge i.v. catheters. DESIGN: A double-blind experiment in four recorded trials was conducted using a rabbit model. Twenty-four-gauge catheters were placed in both auricle veins, and all catheters were secured in the same manner. Ampicillin was infused into each ear at the same interval, with each ear randomized to receive either the heparin solution or the saline solution. SAMPLE: The subjects were ten white New Zealand rabbits. Data were collected on 76 catheter sites. MAIN OUTCOME VARIABLE: Patency for each catheter was measured in hours and evaluated based on presence of warmth, erythema, induration, leakage, or occlusion. RESULTS: There was no significant difference in the duration of patency of the catheters between the two groups.