Reasons for performing the study: As growth hormone increases lean body mass, it could be a therapy for obese horses. However, growth hormone useinduces hyperinsulinaemia in some species, so further investigation is warranted.Objectives: To investigate the effects of feeding, exercise and growth hormone therapy on basal insulin concentrations in healthy horses.Study design: In vivo experimental study.Methods: Blood sampleswere obtained every 30 min from 12 geldings over 24 h, to establish basal serum insulin concentrations, before they underwent a3 week exercise programme. Horses were allocated into 2 groups and exercised for another 4 weeks. Group A received daily i.m. injections of recombinantequine growth hormone; 5 mg/day for 5 days, then 12.5 mg/day for 16 days. Blood sampleswere taken daily before feeding. Insulin vs. time area under curveof Groups A and B were compared using a Student's t-test.Results: Horses demonstrated insulin peaks within 2 h of feeding of 577+/-108.3 pmol/l at 09.30 h and 342.4+/--75.7 pmol/l at 17.30 h, despite receiving thesame meal. The nadir was between midnight and 07.30 h. Exercise had no effect on basal insulin concentrations prior to equine growth hormoneadministrations. The equine growth hormone injections increased serum insulin concentrations (P = 0.01) within Group A, from44.4+/-15.3 pmol/l initially to320.9 +/- 238.2 pmol/l by Day 12. Exogenous growth hormone caused variable hyperinsulinaemia, which was alleviated once equine growth hormoneadministration ceased.Conclusions: Single serum samples taken prior to the morning meal provide basal insulin concentrations. Exercise did not change basal insulinconcentrations. However, equine growth hormone injections increased basal insulin concentrations, which were not ameliorated by exercise.Potential relevance: This therapy is not recommended to address obesity in insulin-resistant equids.