Intravenous glucose tolerance testing (0.5 g/kg of body weight) was done on 2 groups of healthy horses maintained with hay (group 1, n = 5) and with hay plus grain supplementation (group 2, n = 5) and on a group of horses with clinically diagnosed pituitary adenoma (group 3, n = 10). Healthy horses showed an immediate increase of plasma glucose concentration after the IV glucose injection, with return of values to base line in 1 hour. Group 3 horses showed resting hyperglycemia and a delayed return of glucose values to base line (3 hours). Group 3 horses showed resting hyperinsulinemia and a feeble (nonsignificant) response to the glycemic stimulus, with gradual decrease of insulin values to base line. In addition to the apparently reduced tissue sensitivity to insulin in group 3 horses, as evidenced by hyperglycemia, hyperinsulinemia, and protracted glucose and insulin curves, the initial decrease in the insulin/glucose ratio indicates that there was secretory deficiency in response to acute IV glucose loading.