Meal-induced insulin sensitization (MIS) is explained by the HISS (hepatic insulin sensitizing substance) hypothesis. In the presence of two "feeding signals," a pulse of insulin results in the release of HISS from the liver. HISS acts selectively on skeletal muscle and doubles the response to insulin. HISS is not released in the fasted state or in the sucrose-supplemented diabetes model. We tested the hypothesis that provision of both feeding signals allows insulin to cause HISS release in both the normal fasted and the diabetic model. The dynamic response to insulin (50 mU/kg over 5 min) was quantified using the rapid insulin sensitivity test (RIST). Gastric injection of a liquid test meal or i.v. administration of N-acetylcysteine in 24 h fasted rats raised hepatic glutathione to a similar degree (by 46%-47%). Hepatic denervation in fed rats eliminated the parasympathetic signal and eliminated MIS, and bethanechol completely restored MIS. Both compounds administered together allowed insulin to stimulate HISS release in 24 h fasted rats and in a diabetic model (9-week, 35% liquid sucrose supplement). Neither was effective alone. Both "feeding signals" are necessary and sufficient for insulin to stimulate HISS release.