REASONS FOR PERFORMING STUDY: Laminitis in equids is a very common debilitating disease, and insulin resistance (IR) and hyperinsulinaemia are increasingly recognised as important predisposing factors. Pharmacological modification of IR and hyperinsulinaemia might reduce the risk of laminitis. HYPOTHESIS: Metformin, a drug commonly prescribed for treatment of human IR, may also decrease IR in equids. METHODS: Eighteen horses and ponies with IR and recurrent laminitis were treated with 15 mg/kg bwt metformin per os q. 12 h. Each animal served as its own control by comparing pre- and post treatment proxies for IR, insulin sensitivity (IS) and pancreatic beta cell function while controlling for possible dietary and managemental influences on IR. RESULTS: Evidence of significantly improved IS and decreased pancreatic beta cell secretion was found following metformin treatment. The magnitude of effect was greater at earlier resampling (6-14 days) than at later times (23-220 days). Apparent subjective clinical benefits were good but less favourable than effects on IR. CONCLUSIONS: Metformin is safe and appears to increase IS in equids. POTENTIAL RELEVANCE: Metformin may be indicated as a treatment for IR in equids. Further studies are required to define appropriate selection of subjects warranting therapy, dosing schedule and pharmacokinetics.