Bacillus thuringiensis (Bt) transgenic cotton shows unstable resistance to bollworm in China as in other major Bt cotton growing regions of the world.  The objective of this study was to investigate the effects of low temperature stress during critical developmental stages on the leaf CryIA insecticidal protein content and nitrogen metabolism. The study was undertaken on two transgenic cotton cultivars, one conventional (Sikang1) and another hybrid (Sikang 3), during the 2007 and 2008 growing seasons at Jiangsu Provincial KeyLaboratory of Crops Genetics and Physiology, Yangzhou,  China. In the 2007 study, potted cotton plants were exposed to 18°C for 24 hours under glasshouse conditions at three growth stages-peak square, flowering and peak boll developing periods. In 2008 the same two cultivars were exposed to 18°C for 48 hours again at same three growth stages. The results of the study indicated that the leaf insecticidal protein content was significantly affected by the cold treatment at all the three developmental  stages. However, the greatest reduction was observed when the treatment was applied at peak boll period. The CryIA protein content decreased by approximately 23.7% for the cultivar Sikang 1 and 28.1% for Sikang 3 after 24 hours stress in the 2007 study and by approximately 52.9% and 47.6% for 48 hours with the same cultivars, respectively, in the 2008 study. Leaf nitrogen metabolism, measured by the GPT activity, total free amino acid, soluble protein content and the activity of protease, was also significantly reduced by the cold treatments. The changes of nitrogen metabolism closely corresponded to the change of CryIA protein content, suggesting that low temperature may result in the reduction of soluble protein in the leaf, with a resulting decline in the level of the toxin CryIA. It is believed that this may be the cause of the reduced efficacy of Bt cotton in growing conditions in China, where temperatures during the cotton growing season often decrease to 15-20°C.