A bibliographical study showed that increasing supplies of glucogenic nutrients lead to a curvilinear increase in milk and protein yield. Increased post-hepatic glucose availability may be involved in the increase in milk yield. In the present experiment, 5 dairy cows were arranged in a 5 x 5 Latin square design to compare the respective effects of 2 amounts of either duodenal glucose or ruminal propionic acid (C3) on glucose metabolism. Treatment consisted of a grass silage-based diet supplemented with glucogenic nutrients infused into the rumen as a mixture of volatile fatty acids (control) or C3 (6.5 and 13 mol/d) or as glucose (3.4 and 6.9 mol/d) infused into the duodenum. Treatments were isoenergetic and isonitrogenous and contained 100 and 115% of energy and protein requirements, respectively, according to the Institut National de la Recherche Agronomique. Glucose appearance rate (Ra) tended to increase with the level of infusions of both glucogenic materials and with the high dose of duodenal glucose. Plasma insulin-like growth factor-I (IGF-I) concentration increased with the infusion of glucogenic materials compared with the control and was significantly higher with glucose than with C3 treatments. This experiment did not indicate whether the increased Ra was the key mechanism to increased milk yield because milk yield only tended to increase and the standard error for Ra was high. With the high dose of glucose infused into the duodenum, the Ra increase was greater than the increased lactose production in milk. Because of that connection, IGF-I may also be involved by favoring the glucose utilization by the mammary gland.