Fenugreek (Trigonella foenum graecum L) is a plant traditionally used for the treatment of diabetes. It contains an unusual amino acid, 4-hydroxyisoleucine, demonstrated to have insulinotropic and antidiabetic properties in animal models. Here we examine the effect of 4-hydroxyisoleucine on liver function and blood glucose in two rat models of insulin resistance, fructose-fed rats and streptozotocin-induced diabetes type 2. In fructose-fed rats, levels of glucose and liver damage marker aspartate transaminase were markedly (84% and 93%, respectively) and significantly elevated compared with controls (p < 0.001 for both). Alanine transaminase was elevated slightly (18%), and all markers were restored to near control values after treatment with 4-hydroxyisoleucine at 50 mg/kg per day for 8 weeks, the effect being significant (p < 0.01) for all markers. This prolonged exposure to 4-hydroxyisoleucine was well tolerated in control animals and did not alter levels of glucose or liver damage markers significantly. In diabetic rats, treatment with 4-hydroxyisoleucine did not affect glucose or liver damage markers, but did improve HDL-cholesterol levels (31% increase, p < 0.05). These findings indicate 4-hydroxyisoleucine as a useful and well-tolerated treatment for insulin resistance, both directly as a hypoglycaemic and also as a protective agent for the liver. Copyright 2008 John Wiley & Sons, Ltd.