The role of the epidermal cell has rarely been adequately investigated in studies of stomatal movements. In this study of Vicia faba, the changes in aperture in isolated epidermal strips and in intact leaf sections in response to a variety of CO2 concentrations and temperatures in both light and darkness have been observed for guard cells that were in contact with live epidermal cells. The results indicate that the response of the stomatal system to CO2 is largely dependent upon the degree to which the guard cell is functionally connected to the rest of the leaf. Stomata of guard cells in contact with live epidermal cells from leaf sections showed a significant decrease in aperture with increased CO2. Stomata of guard cells in contact with live epidermal cells on isolated epidermal strips, however, showed an increase in aperture with increased CO2 which also appears to be related to temperature. Results can be interpreted as supporting the chemiosmotic hypothesis of stomatal movements. The data also indicate that there are two opposing responses in the stomatal system to CO2: an opening response located in the epidermis and an overriding closing response controlled by the mesophyll.