Serum insulin-like activity was measured serially by the epididymal fat pad method in twelve dogs over several weeks. The animals were first hypophysectomized and subsequently pancreatectomized; no insulin was administered at any stage of the experiment. Hypophysectomy resulted in a reduction of serum ILA of some 50 per cent from an average normal fasting value of 420 μU. per ml.
Subsequent pancreatectomy caused no further appreciable reduction in serum ILA over the entire observation period of five weeks. In a selected number of serum samples residual ILA was detectable also by the rat hemidiaphragin procedure. None of the Houssay dogs had measurable amounts of insulin in blood by immunoassay. It was considered unlikely that activities measured by the two bio-assays represented insulin.