In general, significant decrease in exogenous-toxin (i.e. Bt) protein were shown when transgenic Bt crops (e.g. cotton and rice etc) grown in elevated CO₂ compared with ambient CO₂. So transgenic Bt crops must be faced new ecological risk owing to the decrease in Bt toxin content caused by atmospheric CO₂ level arising. Do increased carbohydrates of transgenic Bt crops, causing by elevated CO₂, dilute the Bt toxin content in plant tissues? A preliminary study was carried out to ascertain the dilution effect hypothesis, by growing transgenic Bt rice (cv. Kemingdao-II expressing Cry1Ab) under ambient and elevated CO₂ in open-top chambers. The dilution effects were shown in under- and above-ground tissues of transgenic Bt rice grown in elevated CO₂ compared with ambient CO₂ for 50 days after planting (i.e. seedling stage) with significant increases in biomass per plant and Bt amount per plant, and significant decreases in Bt content, but wasn't found in leaves, leaf sheathes, shoot or roots for 100 days after planting (i.e. shooting stage). Moreover, cultivation practices, e.g., spraying special hypermethylation reagents (e.g., 5-azaC) to relieve the exogenous-Bt gene expression silencing, and in combination with nitrogen-fertilization supply have been suggested to improve the Bt-gene expression effectively to control target-herbivore pests of transgenic Bt crops (e.g. Helicoverpa armigera for transgenic Bt cotton and Chilo suppressalis for transgenic Bt rice) especially in elevated CO₂.