Indole diterpene alkaloids have been isolated from Ipomoea asarifolia and I. muelleri and are associated with a tremorgenic syndrome in livestock. To better characterize the tremorgenic activity of the major indole diterpene alkaloids in these two plants, terpendole K (1), 6,7-dehydroterpendole A (2), 11-hydroxy-12,13-epoxyterpendole K (3), terpendole C (5), paxilline (6), and a new compound, 6,7-dehydro-11-hydroxy-12,13-epoxyterpendole A (4), were isolated and evaluated for tremorgenic activity in a mouse model. Compounds 1, 2, 5, and 6 all showed similar and significant signs of tremorgenic activity. In contrast, the 11-hydroxy-12,13-epoxy compounds, 3 and 4, showed no significant tremorgenic activity.