This investigation was concerned with the taxonomic relationship between Astragalus beckwithii (Torr. & Gray) var. beckwithii, Astragalus oophorus (S. Wats) var. caulescens (Jones) Jones, and Astragalus megacarpus (A. Gray). An attempt was made to relate some chemical constituents and the toxic effects on rats to the taxonomy. A number of parameters were used; rat toxicity, lipid analysis, selenium atomic absorption, various salts and glucose percentages, as well as the usual morphological measurements taken from herbarium specimens, field observations, and pollen measurements. While the weight of the data indicates that A. beckwithii is a type of intermediary or intergressory product of A. oophorus and A. megacarpus, and that it very closely resembles A. oophorus, it would be premature to suggest combining A. beckwithii and A. oophorus until the related varieties in each species are also studied. Evidence shows definite symptoms of locoism in the white rats tested. A hypothesis was made that the toxic principles in these plants are associated with the nitrile compounds.