To test the hypothesis that glucagon will decrease the severity of fatty liver and prevent ketosis, glucagon was continuously infused via intravenous jugular catheters into dairy cattle in several physiological states. In preliminary dosage response studies, glucagon was infused into eight spayed Holstein heifers, four midlactation Holstein cows, and four early-lactation Brown Swiss cows in cross-over design trials at dosages of 20 mg/d or less for 48 h. Plasma glucose concentrations were increased by glucagon in a dose-dependent manner. Plasma nonesterified fatty acid and ketone body concentrations were not increased by glucagon at 5 mg/d or less, were increased in heifers at 10 mg/d, and increased in cows at 20 mg/d compared with preinfusion concentrations. Glucagon at 10 mg/d was the largest dosage tested in lactating cows that did not elicit a lipolytic or ketogenic response. Glucagon at 10 mg/d or vehicle then was infused for 14 d during a protocol to induce ketosis in 20 early-lactation Holstein cows already having fatty livers. Plasma glucose concentrations were increased by 42% by glucagon. Insulin concentrations increased during the first 4 h of glucagon infusion, thereafter they were not different than concentrations in plasma of control cows. Concentrations of nonesterified fatty acids and ketone bodies in plasma were decreased over time by glucagon compared with concentrations in plasma of control cows. In livers of cows treated with glucagon, concentrations of triacylglycerols rapidly and dramatically declined. At the end of glucagon treatment, triacylglycerol concentrations in livers of glucagon-treated cows had been decreased by 71% compared with livers of control cows. Glucagon caused liver glycogen concentrations to transiently decrease and ultimately increase to 231% of the glycogen concentrations in livers of control cows. Production of milk and milk protein was decreased during glucagon treatment but rapidly increased to levels of production by control cows after treatment. No adverse effects of glucagon were observed on the health of the cows. In summary, glucagon can be used as an effective treatment for fatty liver and increasing the resistance of early-lactation cows to ketosis.