Technical Abstract: Locoweed consumption by livestock decreases animal performance and can cause death. Swainsonine, the principle toxic alkaloid in locoweed, acts by inhibiting alpha-mannosidase which affects oligosaccharide metabolism. Primiparous cows (n=16) were used to evaluate the correlation between locoweed consumption (as measured by bite counts) and serum swainsonine, alpha-mannosidase, and aspartate aminotransferase (AST), a liver enzyme indicative of mitochondrial damage. The heifers were randomly divided into two groups and fed either grass hay or alfalfa hay for 70 d prior to exposure to locoweed infested pastures to determine whether legume hay affected locoweed-feeding preference. There was no difference (P>.1) in locoweed preference. Blood samples were collected weekly to determine the level of swainsonine, AST, and alpha- mannosidase activity. Bite counts were recorded twice a day, 5 days per week, for 5 weeks on each animal. The concentration of swainsonine in the serum was positively correlated (P<.05) with the weekly locoweed bites (r=.645) and with AST concentrations (r=.415). Serum swainsonine concentrations were not correlated (P>.1) to alpha-mannosidase activity (r=-.051). Also, alpha-mannosidase was not correlated (P>.1) to either bite counts (r=-.082) or AST values (r=-.15). Legume feeding does not appear to predispose cows to eating locoweed. Bite counts appear to accurately reflect serum swainsonine and AST concentrations, but not alpha-mannosidase activity. The alpha-mannosidase activity seemed low at the beginning of this study and remained low. It is possible that serum swainsonine concentrations were not elevated for a sufficient length of time to depress alpha-mannosidase activity.