Sunflower (Helianthus annuus L., cv. NK894) seedlings, grown hydroponically, were exposed to ozone concentrations of 0.0, 0.1, 0.2, and 0.3 ppm at 60% RH, or 0.0 and 0.3 ppm at 30% and 90% RH, for 6 h/d, for four consecutive days, at ages 15 to 18 days from seeding. Plant response was detected through measurement of leaf growth rate, and plant dry weight, as well as visible foliar injury. Ozone flux was estimated by a model based on Fick's Law of diffusion, with the assumption that ozone and water vapor experience equal diffusion resistances, and follow the same diffusion pathways. Visible foliar injury, reductions in leaf growth rate, and reductions in dry weight were highly correlated with predicted ozone flux. Low ozone flux induced a significant change in plant biomass partitioning -- an increase in the proportion of total plant dry weight comprised by the leaves, and a reduction in that comprised by the roots -- accompanied by a significant reduction in root, but not total plant dry weight. Visible foliar injury and significant reductions in total plant dry weight were not induced until ozone flux was slightly higher. Significant reductions in leaf growth rate (except at 30% RH), and stem and leaf (except at 30% RH) dry weight occurred only at the highest ozone fluxes, although growth rate was strongly correlated with low, as well as high ozone fluxes. Ozone flux and plant response to ozone were greater at high, than at low relative humidity. Similar rates of growth and biomass reduction between high (90%) and low (30%) relative humidities suggested that the amount of reduction per unit ozone flux was unaffected by the level of relative humidity. This would support previous evidence that increased plant response to ozone at increasing relative humidity is due to decreased stomatal, rather than internal leaf resistance.