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Freshly harvested caryopses of Hordeum spontaneum do not germinate, even after a long period of imbibition at 10, 20 and 30 degrees C, in darkness or in continuous white light. Their dormancy results from an inhibitory effect of the covering structures, since isolated embryos are able to germinate. Breaking of dormancy, termed ’after-ripening’, occurs during storage in dry conditions at 35 degrees C or in the natural habitat during summer. Dormancy remains when caryopses in dispersal units are stored at -35 degrees C. The ecological importance of dormancy and after-ripening at high temperatures affecting the ’readiness for germination’ is discussed. (C) 1996 Academic Press Limited