At harvest, barley seeds are dormant because their germination is difficult above 20°C. Incubation of primary dormant seeds at 30°C, a temperature at which they do not germinate, results in a loss of their ability to germinate at 20°C. This phenomenon which corresponds to an induction of a secondary dormancy is already observed after a pre-treatment at 30°C as short as 4–6 h, and is optimal after 24–48 h. It is associated with maintenance of a high level of embryo ABA content during seed incubation at 30°C, and after seed transfer at 20°C, while ABA content decreases rapidly in embryos of primary dormant seeds placed directly at 20°C. Induction of secondary dormancy also results in an increase in embryo responsiveness to ABA at 20°C. Application of ABA during seed treatment at 30°C has no significant additive effect on the further germination at 20°C. In contrast, incubation of primary dormant seeds at 20°C for 48 and 72 h in the presence of ABA inhibits further germination on water similarly to 24–48 h incubation at 30°C. However fluridone, an inhibitor of ABA synthesis, applied during incubation of the grains at 30°C has only a slight effect on ABA content and secondary dormancy. Expression of genes involved in ABA metabolism (HvABA8′OH-1, HvNCED1 and HvNCED2) was studied in relation to the expression of primary and secondary dormancies. The results presented suggest a specific role for HvNCED1 and HvNCED2 in regulation of ABA synthesis in secondary seed dormancy.