Objective - To evaluate the effects of endotoxin administered IV on glucose and insulin dynamics in horses. Animals - 16 healthy adult mares. Procedures - Each week of a 2-week randomized crossover study, each horse received an IV injection (duration, 30 minutes) of Escherichia coli O55:B5 lipopolysaccharide (LPS) in 60 mL of sterile saline (0.9% NaCl) solution (20 ng/kg) or sterile saline solution alone (control treatment). Frequently sampled IV glucose tolerance test procedures were performed at 24 hours before (baseline) and 24 and 48 hours after injection; glucose and insulin dynamics were assessed via minimal model analysis. Results - 13 of 16 horses had a clinical response to LPS, which was characterized by mild colic and leukopenia. Before treatment, mean ± SD insulin sensitivity was 2.9 ± 1.9 × 104 L•min-1•mU-1; this significantly decreased to 0.9 ± 0.9 × 10-4 L•min-1•mU-1 24 hours after treatment (69% reduction) and was 1.5 ± 0.9 × 10-4 L•min-1•mU-1 48 hours after treatment. At baseline, mean ± SD acute insulin response to glucose was 520 ± 196 mU•min•L-1; this significantly increased to 938 ± 620 mU•min•L-1 (80% increase) and 755 ± 400 mU•min•L-1 (45% increase) at 24 and 48 hours after LPS treatment, respectively. Conclusions and Clinical Relevance - Compared with baseline values, insulin sensitivity was decreased for 24 hours after IV injection of LPS, and affected horses had a compensatory pancreatic response. These disturbances in glucose and insulin dynamics may contribute to development of laminitis in horses.