Araucaria angustifolia is a critically endangered species and its distribution can be affected by an increase in temperature. In this study, we evaluated the effects of heat stress (30°C) on Araucaria angustifolia cell lines responsive (SE1) and non-responsive (SE6) to the development of somatic embryos. The viability of both cell lines was reduced by heat stress and mitochondria were the organelles most affected. Heat stress for 24h increased the reactive oxygen species (ROS) levels in SE1 cells, followed by a reduction at 48 and 72h. In SE6 cells, an increase occurred after 24 and 48h of stress, returning to control levels at 72h. H2 O2 levels were increased after 24h for both SE1 and SE6 cells, being higher for SE6. Interestingly, at 48 and 72h, H2 O2 levels decreased in SE1 cells, while in SE6, the values returned to the control levels. The respiration of SE6 cells in the presence of oxidisable substrates was inhibited by heat stress, in agreement with the high lipid peroxidation levels. The AaSERK1 gene was identified in both cultures, with greater expression in the SE1 line. Heat stress for 24 and 48h increased gene expression only in this cell line. The activity of peroxidase, superoxide dismutase and enzymes of the glutathione/ascorbate cycle was increased in both cell lines subjected to heat stress. Catalase activity was increased only in SE6 cells at 72h of exposure. These results show that responsive SE1 cells can modulate ROS levels more efficiently than SE6 when these cells are stressed by heat. This ability may be related to the maturation capacity of these cells.