Insulin-like growth factor-I (IGF-I) has been demonstrated to enhance mucosal repair following intestinal damage induced by chemotherapeutic agents (intestinal mucositis). However, the potential for prophylactic IGF-I to protect the intestine remains undefined. We investigated the effects of IGF-I pre-treatment on chemotherapy-induced mucositis in rats. Male Sprague Dawley rats were treated for 7 days with 0 or 4.3mg/kg/day IGF-I delivered systemically via osmotic mini-pump. Rats received an intraperitoneal injection of 0 or 150 mg/kg 5-fluorouracil (5-FU) on day 7 and were killed 48 h later for assessment of intestinal damage and repair. Compared to normal controls, 5-FU decreased epithelial proliferation by 86%, concurrently increasing the incidence of apoptosis 87-fold, whilst decreasing small intestinal (SI) length by 14%, SI weight by 30% and total gut weight by 24%. 5-FU decreased villus height in the duodenum (23%), jejunum (20%) and ileum (30%) with crypt depths decreased by 31%, 27% and 33% in these gut regions. These effects were less profound in IGF-I pre-treated rats in which apoptosis was increased 48-fold, with SI length decreased by 7%, SI weight by 18% and total gut weight by 15% accompanied by decreases in villus height of 8% (duodenum), 14% (jejunum) and 21% (ileum), and crypt depth decreases of 23%, 16% and 17% for the same gut regions, compared to normal controls. We conclude that IGF-I pre-treatment only partially attenuates features of intestinal mucositis when assessed 48 h after 5-FU chemotherapy.