The mechanism of seed dormancy at low temperatures (15-9°C) was investigated in the seeds of Syringa josikaea, S. reflexa and S. vulgaris. Low temperature dormancy in Syringa species was mainly imposed by endosperm embedding the radicle. Different degrees of embryo dormancy may occur in S. reflexa seeds. In most cases the low temperature dormancy was broken completely by removing the endosperm around the radicle. The endosperm did not seem to contain significant quantities of germination inhibitors, and the results indicate that it prevents germination mainly due to its mechanical resistance. The mechanical resistance of endosperm did not change during chilling or during induction of dormancy by high temperature incubation. The strength of the endosperm decreased rapidly in non-dormant seeds before visible germination. Similar changes were not observed in dormant seeds. Generally, the strength of the endosperm was lower in the non- (or less) dormant species S. josikaea and S. vulgaris than in the more dormant S. reflexa seeds. The growth potential of the embryos, measured as their ability to germinate in osmotic solutions (mannitol or polyethylen glycol 4000), was increased by chilling and by GA3-treatment. The growth potential of untreated S. josikaea and S. vulgaris embryos was generally higher than that of S. reflexa embryos. Acid ethyl acetate fractions of methanol extracts from embryos of all three species contained substances with GA3-like activity in the lettuce hypocotyl test. The activity was found at Rf 0.9–1.0 on paper chromatograms run in distilled water. No significant changes in the activity were detected during chilling or prior to visible germination.