Abstract A neurological condition resembling that observed in hereditary mannosidosis occurs in animals ingesting spotted locoweed and plants of the genus Swainsona. Swainsonine has been isolated from these plants and has been suggested to be the primary causative agent in inducing the pathological condition. This alkaloid has also been found to increase tissue acid α- d-mannosidase levels in rats while lowering liver Golgi mannosidase II levels. In the present study, the effects of locoweed and swainsonine were directly compared for the first time, with the pig as experimental animal. Both increased most lysosomal acid glycosidase activities in most tissues, decreased liver Golgi mannosidase II levels, increased plasma hydrolase levels, and greatly increased tissue oligosaccharide, especially Man 5GlcNAc 2 and Man 4GlcNAc 2. These results indicate that swainsonine is the agent in locoweed responsible for the enzymatic and oligosaccharide changes. The behavior of the animals was also similarly affected by swainsonine and locoweed.