Gluconeogenesis has been studied in perfused livers of normal albino, as well as of C57 lean and obese-hyperglycemic ( ob/ob) mice. In livers of normal albino mice, the highest rate of gluconeogenesis was observed with saturating levels of fructose and dihydroxyacetone. Pyruvate and lactate were about half as effective. Oleate-albumin complex did not stimulate gluconeogenesis. Glucagon, epinephrine or cyclic AMP did not alter gluconeogenesis, despite the fact that they were strongly glycogenolytic, suggesting that gluconeogenesis in livers of normal albino mice is regulated mostly by substrate availability. Gluconeogenesis in livers of C57 lean and ob/ob mice was compared and it was found that the capacity for gluconeogenesis from lactate or pyruvate was similar in both groups of mice. Lipogenesis from glucose was studied in C57 lean and ob/ob mice, using an in vivo technique. Lipogenesis per total liver or per total adipose tissue was extremely high in the ob/ob animals compared with the lean ones. Lean mice responded to intravenous insulin with a marked increase in lipogenesis while ob/ob mice did not. Insulin-stimulated lipogenesis in the lean animals was similar in magnitude to that of non-treated ob/ob mice. When ob/ob mice were made relatively insulin deficient by treatment with streptozotocin or anti-insulin serum, liver and adipose tissue lipogenesis were markedly decreased. It is postulated that part of the increased lipogenesis seen in the ob/ob mice may be related to their abnormally high circulating insulin levels. This view is strengthened by experiments carried out on perfused livers of C57 lean and ob/ob mice. Basal and substrate-stimulated lipogenesis, as well as newly synthesized triglyceride secretion by livers of ob/ob mice, was greater than that of lean controls. Furthermore, in the absence of oleate, the secretion of triglycerides by livers of ob/ob mice was much higher than that of lean mice but it was not stimulated by oleate, while such a stimulation could be obtained in livers of normal controls. All these anomalies of lipid metabolism could be restored towards normal when the ob/ob mice were made relatively insulin deficient by streptozotocin treatment prior to the experiments. In perfused livers of normal albino mice the release of triglycerides into the perfusate has been measured as an index of very low density lipoprotein (VLDL) secretion. Vincristine or colchicine, drugs known to interfere with the microtubular system, were found to inhibit markedly triglyceride release. Biochemical and ultrastructural evidence has indicated that these drugs affect specifically microtubules. In a similar fashion, vincristine was found to decrease the secretion of globulins, albumin and small polypeptides by perfused livers. It is concluded that the functional integrity of microtubules is important for the intracellular movement and eventual release from the liver of VLDL particles, as well as of albumin and other proteins.