Genetic variation for high temperature tolerance exists in wheat cultivars. Identifying key metabolic steps in this regard will help in improving grain yield. Effect of high temperature on activities of sucrose metabolizing enzymes and aminotransferases in relation to transformation of free sugars to starch and protein in developing grains of four wheat cultivars PBW 343, PBW 550, C 306, C 273 was studied by raising the crop under normal (October) and late planting (December) conditions. Significant reduction in chlorophyll content and increase in membrane injury index and lipid peroxidation were observed in all the cultivars. C 306 and C 273 showed higher membrane stability over PBW 343, PBW 550. High temperature accentuated sucrolytic enzymes and aminotransferases while activities of sucrose synthase/phosphate synthase got repressed. Higher build up of sugars and starch in PBW 343, PBW 550 cvs over C 306, C 273 resulted in higher yield in the former but their lower membrane integrity under heat stress leads to yield reductions. Disruption of starch biosynthesis resulted in their favoured utilization in nitrogen metabolism leading to higher amino acid and protein accumulation. Results indicated that sustainability for yield enhancement under stress could be possible by cross-breeding high yielding cvs PBW with increased membrane stability C ones for increasing thermotolerance. Grain filling processes in terms of sugars/starch were strongly correlated to sucrose synthase/phosphate synthase activities while protein to aminotransferases.