Two herbaceous perennials, alfalfa (Medicago sativa L. cv. Arc) and orchard grass (Dactylus glomerata L. cv. Potomac), were grown at ambient (367 μmol mol−1) and elevated (729 μmol mol−1) CO2 concentrations at constant temperatures of 15, 20, 25 and 30°C in order to examine direct and indirect changes in nighttime CO2 efflux rate (respiration) of single leaves. Direct (biochemical) effects of CO2 on nighttime respiration were determined for each growth condition by brief (<30 min) exposure to each CO2 concentration. If no direct inhibition of respiration was observed, then long-term reductions in CO2 efflux between CO2 treatments were presumed to be due to indirect inhibition, probably related to long-term changes in leaf composition. By this criterion, indirect effects of CO2 on leaf respiration were observed at 15 and 20°C for M. sativa on a weight basis, but not on a leaf area or protein basis. Direct effects however, were observed at 15, 20 and 25°C in D. glomerata; therefore the observed reductions in respiration for leaves grown and measured at elevated relative to ambient CO2 concentrations could not be distinguished as indirect inhibition. No inhibition of respiration at elevated CO2 was observed at the highest growth temperature (30°C) in either species. CO2 efflux increased with measurement and growth temperature for M. sativa at both CO2 concentrations; however, CO2 efflux in D. glomerata showed complete acclimation to growth temperature. Stimulation of leaf area and weight by elevated CO2 levels declined with growth temperature in both species. Data from the present study suggest that both direct and indirect inhibition of respiration are possible with future increases in atmospheric CO2, and that the degree of each type of respiratory inhibition is a function of growth temperature.