Galphimia glauca has been used in Mexican traditional medicine as a remedy for the treatment of nervous excitement and other central nervous system disorders. Previous work has demonstrated the sedative, spasmolytic and anticonvulsant activities of an extract obtained from the aerial parts of this plant. A norsecotriterpene named galphimine B was found to be responsible for the sedative and spasmolytic activities. Activity-guided fractionation making use of the guinea pig ileum as an experimental system was used for the isolation of five compounds. They were identified as a new norsecotriterpene, galphimine J (2) and four known norsecotriterpenes (1, 3 - 5). Nine derivatives of 3 (5, 6 and 8 - 14) were obtained by chemical modification of the α,β-unsaturated lactone, the olefinic bonds on the A and E rings, the hydroxy groups on ring B, or a combination thereof, and a structure-activity correlation study was carried out. The results indicate the existence of a clear structure-activity relationship. They suggest the involvement of the double bond in the E ring as well as the hydroxy groups at C-4, C-6 and C-7 in biological activity and emphasize the relevance of a free OH group at C-6 and C-4.