Maturation of Acer pseudoplatanus seeds comprises three phases. The first is a growth phase lasting for about 100 d, during which fresh weight and dry weight increase steadily. The end of the growth phase is marked by a temporary cessation of weight increase and is followed by a phase of reserve accumulation extending for a further 60 d. During this phase levels of extractable lipid, soluble and insoluble carbohydrate, protein, and membrane-bound ribosomes show a marked increase. The third phase is a period of desiccation, lasting at least 30 d. The kind of dormancy exhibited by the immature seed is characteristic of the stage of maturation. Before the desiccation phase the embryo has no capacity for germination even when subjected to treatments known to promote germination in the mature seed. In the desiccation phase the seed becomes responsive to chilling, but a wide range of hormones and other substances known to promote seed germination are ineffective. Embryos contain a persistently high level of acidic inhibitors throughout maturation. The level of neutral inhibitors, initially high, showed a marked drop after 150 d and remained low for the rest of the maturation period.