Elevated temperature often negatively affects crop growth and development, resulting in impaired reproductive growth and lower seed yields. A study was conducted to investigate the vegetative and reproductive growth responses of soybean (Glycine max L.) cv. Maverick (MG III, indeterminate) by increasing temperatures at two oxygen levels (21% and 32%) in a controlled greenhouse environment. Carbon dioxide concentration was held constant at 700 ppm to minimize photorespiration. Seed were sown in pots on 16 May 2011 and grown at day/night temperatures of 30/22 degrees C in eight rooms of the controlled environment greenhouse for 21 days. Four day/night temperature regimes were established at 21 days for the duration to harvest maturity: 30/22 (control, “optimal” regime), 34/26, 38/30 and 42/34 degrees C. Flowering (R1) occurred at 24 days after sowing. Results indicated that elevated oxygen had no significant effect on development rate and had a non-significant tendency for a reduction in biomass productivity. Therefore, results were averaged over oxygen treatments. Supra-optimal temperatures accelerated soybean vegetative growth. Plants grown at higher temperatures had more nodes, greater plant height (except at 42/34 degrees C), increased plant leaf area (except at 42/34 degrees C), increased leaf dry weight, and increased stem dry weight (except at 42/34 degrees C). In addition, supra-optimal temperature slowed reproductive development (reproductive stage progression), as well as decreased pod number, pod weight per plant, and pod harvest index. The highest temperature regime (42/34 degrees C) inhibited both vegetative growth and reproductive development, leading to the formation of the smallest plants with the lowest leaf area and a failure to develop pollen, with zero seed yield. We conclude that supra-optimal temperature shifted the balance from reproductive to vegetative development. Understanding the processes of impaired reproductive development and growth is important for ameliorating the effects of global climate change.