Two new secoiridoid glucosides, excelsides A (1) and B (2), were isolated from the seeds of Fraxinus excelsior. Their structures were elucidated as (2S,4S,3E)-methyl 3-ethylidene-4-(2-methoxy-2-oxoethyl)-2-[(6-O-beta-D-glucopyranosyl-beta-d-glucopyranosyl)oxy]-3,4-dihydro-2H-pyran-5-carboxylate and (2S,4S,3E)-methyl 3-ethylidene-4-{2-[2-(4-hydroxyphenyl)ethyl]oxy-2-oxoethyl}-2-[(6-O-beta-d-glucopyranosyl-beta-d-glucopyranosyl)oxy]-3,4-dihydro-2H-pyran-5-carboxylate, respectively, on the basis of NMR and MS data. Eight known compounds were identified as nuzhenide (3), GI3 (4), GI5 (5), ligstroside (6), oleoside 11-methyl ester (7), oleoside dimethyl ester (8), 1'''-O-beta-D-glucosylformoside (9), and salidroside (10). Compounds 1-9 inhibited adipocyte differentiation in 3T3-L1 cells. Dilutions of the aqueous extract of F. excelsior (1:10,000) as well as compounds 2, 3, 4, 5, and 8 activated the peroxisome proliferator-mediated receptor-alpha (PPARalpha) reporter cell system in the range of 10(-4) M, compared to 10(-7)-10(-8) M for the synthetic PPARalpha activator, WY14,643. Both biological activity profiles support the hypothesis that inhibition of adipocyte differentiation and PPARalpha-mediated mechanisms might be relevant pathways for the antidiabetic activity of F. excelsior extract.