Phytochemical investigations on the whole plant of Hippobroma longiflora (L.) G. Don have resulted in the isolation and elucidation of three new diphenethylpiperidine alkaloids, hippofoline A (1), hippofoline B (2), and (-)-cis-2',2 ''-diphenyllobelidiol N-oxide (5), alongside two known alkaloids, (-)-lobeline (3) and (-)-cis-2',2 ''-diphenyllobelidiol (4). The absolute configurations of alkaloids 1 and 2 were deduced based on experimental and calculated ECD data as well as biogenetic consideration, while the absolute configurations of alkaloids 3 and 4 in their free base form were confirmed by Cu K alpha radiation X-ray diffraction analyses. Alkaloids 2-4 were shown to induce concentration-dependent relaxation effects on rat isolated tracheal rings that were pre-contracted with carbachol.