A continuous flow system was used to study the interactions between carbon dioxide and ethylene in intact sunflower (Helianthus annuus L.) plants. An increase in the concentration of carbon dioxide above the ambient level (0.033%) in the atmosphere surrounding the plants increased the rate of ethylene production, and a decrease in carbon dioxide concentration resulted in a decrease in the rate of ethylene production. The change in the rate of ethylene production was evident within the first 15 minutes of the carbon dioxide treatment. Continuous treatment with carbon dioxide was required to maintain increased rate of ethylene production. The rate of carbon dioxide fixation increased in response to high carbon dioxide treatment up to 1.0%. Further increases in carbon dioxide concentration had no additional effect on carbon dioxide fixation. Carbon dioxide concentrations higher than 0.11% induced hyponasty of the leaves whereas treatment with 1 microliter per liter ethylene induced epinasty of the leaves.