Phosphatidylinositol transfer proteins (PITPs) regulate the interface between lipid metabolism and cellular functions, but the fundamental nature of this regulation is not understood. To address these questions in mammals, we generated mice nullizygous for various PITP isoforms. Ablation of murine PITPα function leads to aponecrotic spinocerebellar disease, hypoglycemia, and intestinal and hepatic steatosis. The conditions of steatosis suggest defective biogenesis and trafficking of lipoproteins from the endoplasmic reticulum into the circulation. The hypoglycemia is associated with reduced numbers and general compromise of pancreatic islets. These defects lead to reduced proglucagon gene expression and inappropriate channeling of glucose into glycogen stores. The available data indicate hypoglycemia is a major contributing factor in determining rate of onset of spinocerebellar disease. The collective data suggest an unanticipated role for PITPα in mammalian endoplasmic reticulum functions involved in the packaging and transport of specific lumenal lipid cargos.