The effects of alloxan, an insulin (INS)-secreting cell toxin, and INS immunoneutralization on circulating levels of thyroid hormones (thyroxine, T4; triiodothyronine, T3) were examined in larval landlocked sea lampreys, Petromyzon marinus. Animals were injected intraperitoneally with either (Experiment 1) saline (0.6%) or alloxan (20 or 200 μg/g body weight), or with (Experiment 2) normal rabbit serum or anti-lamprey INS. Alloxan (200 μg/g) decreased plasma T3, but not T4, in larvae by about 45-80%. Three, six, or nine hr after acute immunoneutralization of lamprey INS with anti-lamprey INS, plasma T3 levels decreased by 13-30%, relative to controls. These data indicate that INS deficiency can regulate the thyroid system of larval lampreys. There is some suggestion that INS may mediate the metamorphic processes by modulating thyroid hormone concentrations.