External morphology and anatomy of the flower and pollen of Bretschneidera sinensis Hemsl. are described to clarify the position of the family Bretschneideraceae relative to the Sapindales and the glucosinolate-producing families. Anatomical and micromorphological characters are investigated and sections are used to understand the structure of the flower. Observation of buds and sections reveal that the flower is obliquely monosymmetric, with the symmetry line running from one petal to a sepal. The upper petal shields the stamens and pistil and becomes positioned apically by the partial resupination of the pedicel. The octomerous androecium is characterized by variable empty positions which are related to the variable insertion of the three carpels. The loss of stamens is linked with a displacement of the remaining stamens. Floral anatomy demonstrates the presence of a nectary extending on the hypanthium from the base of the filaments to the base of the gynoecium. Details of floral anatomy are compared with members of Sapindaceae, Hippocastanaceae, Moringaceae, Akaniaceae, Tropaceolaceae and Capparaceae. Comparison with other characters supports a close relationship with Akaniaceae and Tropaeolaceae in an order Tropaeolales, in concordance with macromolecular results, either at the base of the glucosinolate clade, or in remote connection with the Sapindales. A number of floral anatomical characters with a strong phylogenetic signal are highlighted. (C) 2002 The Linnean Society of London.