C57B1/6 male normal mice were treated with 5-FU (200 mg/kg i.p.) alone or in combination with a bolus injection of uridine (2 x 3500 mg/kg i.p.) in order to study the potential rescue effect of uridine on 5-FU-induced gastrointestinal toxicity. 5-FU alone inhibited the activity of different enzymes (thymidine-kinase, alkaline-phosphatase, sucrase and maltase) which were selected as the early biochemical markers for the injured small intestinal mucosa. The nadir of the enzyme activities was between 24-96 hrs after 5-FU administration, and the complete regeneration took a week. In the combination of 5-FU plus uridine bolus injection the seriousness of gastrointestinal damage caused by 5-FU was significantly (p < 0.05) milder and the recovery time was shorter by 2 days. Comparing the rescue effect of two dose schedules of uridine, both high dose (2 x 3500 mg/kg) or repeated lower doses of uridine (7 x 800 mg/kg) resulted in a similar protection from the gastrointestinal side effect of 5-FU.