Respiration rates of roots of loblolly pine (Pinus taeda L.) seedlings decreased in response to elevated ozone (O3 ) concentrations. Estimated annual root respiration rates were 12% less in seedlings exposed to twice-ambient O3 (7 h mean = 70.110 nl 1-1 ) than in seedlings exposed to sub-ambient O3 (7 h mean = 20.40 nl 1-1 ). Measurements taken during periods of relatively rapid and of negligible root growth suggest that the reduced root respiration may be due to both reduced maintenance respiration and reduced growth respiration. Respiration rates of the soil substrate of the O3 -exposed seedlings were also below those of the substrate of seedlings exposed to sub-ambient O3 . The study supports the theory that there is a reduced supply of photosynthates to the roots of plants exposed to elevated O3 . It is hypothesized that a reduced supply of photosynthates to the roots may affect soil respiration rates by reducing root exudation rates and, consequently, reducing rhizosphere microbial populations.