We examined the effects of insulin (INS) and somatostatin 14 (SS-14) on glycemic regulation in larval and metamorphosing landlocked sea lamprey, Petromyzon marinus. Animals were injected intraperitoneally with either (Experiment 1) saline (0.6%), somatostatin-14 (SS-14; 50 or 500 ng/g body weight), insulin (INS; 10 or 100 ng/g body weight), or alloxan (20 or 200 μg/g body weight), or with (Experiment 2) normal rabbit serum, anti-SS-14, anti-lamprey SS-34 or anti-lamprey INS. Somatostatin-14 (500 ng/g) increased plasma glucose levels in larvae. Injection of anti-SS-14 and anti-lamprey SS-34 resulted in hypoglycemia compared to the controls. Insulin (100 ng/g) resulted in hypoglycemia in both larvae and stage 6 metamorphosing lamrpey. Acute insufficiency of lamprey INS in larvae treated with anti-lamprey INS elevated plasma glucose levels. Similarly, alloxan (200 μg/g, a cytotoxin of insulin-secreting cells) resulted in hyperglycemia in larvae. These data indicate that SS-14 is hyperglycemic in sea lamprey, whereas INS is hypoglycemic, and suggest that the glucoregulatory roles of SS-14 and INS emerged early during the evolution of vertebrates.