P atients with autoantibodies to the insulin receptor (Anti-R) may exhibit either fasting hypoglycemia or hyperglycemia and extreme insulin resistance. Occasionally, both these phenomena are observed in the same patient at different times in the clinical course. In an effort to understand what determines the patient's response to Anti-R, we developed an animal model of these clinical disorders by passive transfer of Anti-R IgG to rats. IgG fractions from the plasma of Anti-R patients and control subjects were prepared by affinity chromatography with staphylococcal protein A-Sepharose. Anti-R IgG, injected into fasting rats, induced severe and persistent hypoglycemia (plasma glucose 30-60 mg/dl). Rats injected with control IgG maintained a plasma glucose within the range of 75 (fasting) to 165 mg/dl (feeding). In comparison with the effects of insulin, the hypoglycemic response to Anti-R IgG had a slower onset (2-4 h) and lasted longer (8-24 h). Similar, dose-dependent hypoglycemic responses were observed in rats whether the Anti-R IgG was derived from an insulin-resistant or hypoglycemic patient. When Anti-R IgG was administered in sufficiently high doses for several days to fed rats, persistent hyperglycemia (plasma glucose 200-400 mg/dl) developed. Based on these in vivo and previous in vitro studies, we attribute the hypoglycemic response to an insulin-like effect of Anti-R, and the hyperglycemic response to a desensitization of host tissues to the effects of insulin, with more prolonged exposure to higher levels of Anti-R.