In barley, primary dormant grains did not germinate at 30 °C in air and at 15 °C in an atmosphere containing less than 10% O2, while they germinated easily at 15 °C in air. O2 tension in embryos measured with microsensors was 15.8% at 15 °C but only 0.3% at 30 °C. Incubation of grains at 30 °C is known to induce secondary dormancy in barley, and it was shown here that secondary dormancy was also induced by a 3 d treatment in O2 tensions lower than 10% at 15 °C. After such treatments, the grains lost their ability to germinate subsequently at 15 °C in air. During seed treatment in 5% O2, embryo abscisic acid (ABA) content decreased more slowly than in air and was not altered after transfer into air. Hypoxia did not alter the expression of ABA metabolism genes after 1 d, and induction of HvNCED2 occurred only after 3 d in hypoxia. Embryo sensitivity to ABA was similar in both primary and hypoxia-induced secondary dormant grains. Gibberellic acid (GA) metabolism genes were highly regulated and regulated earlier by the hypoxia treatment, with major changes in HvGA2ox3, HvGA3ox2 and HvGA20ox1 expression after 1 d, resulting in reduced GA signalling. Although a high temperature has an indirect effect on O2 availability, the data showed that it did not affect expression of prolyl-4-hydroxylases and that induction of secondary dormancy by hypoxia at 15 °C or by high temperature in air involved separate signalling pathways. Induction by hypoxia at 15 °C appears to be more regulated by GA and less by ABA than the induction by high temperature.