The propose of this study was to evaluate the mobilization and translocation of endosperm reserve substances to roots, shoots and substances removed by respiration during germination and early growth of P. 427 hybrid maize seedlings depending on the use of Reglalg and the impact of heat shock (HS). The use of HS with superoptimal temperatures led to a decrease in seed germination, which in the control (26°C) was 98%, and in the variants with HS of + 44°С, +47°С, + 50°С and +53°C with duration of 30 min. it was 96, 90, 81 and 71%, respectively. It was shown that with an increase in the intensity of HS, the proportion of reserve substances consumed from the endosperm for germination and growth of seedlings decreases, and the proportion of unused endosperm, on the contrary, increases with an increase in HS values from 71.1% in the control to 85% in HS of +53 °C. Utilization of the optimal dose (1/200) of Reglalg preparation for seed treatments before exposure to HS had a beneficial effect on seed germination, reducing the inhibitory effect of HS, and also contributed to an increase in the mass of seed endosperm consumed in the growth processes of seedlings under the influence of HS conditions.