One-year old loblolly pine(Pinus taeda L.) seedlings were grown in an unshaded greenhouse for seven months under four levels of ultraviolet-B(UV-B) radiation simulating stratospheric ozone reductions of 16, 25, and 40% and included a control with no UV-B radiation. Seedlings receiving the lowest supplemental UV-B irradiance showed reductions in growth and photosynthetic capacity after only one month of irradiation. These reductions persisted and resulted in lower biomass production, while no increases in UV-B-adsorbing compounds in needles were observed. Seedlings receiving UV-B radiation which simulated a 25% stratospheric ozone reduction showed an increase in UV-B-absorbing-compound concentrations after six months, which paralleled a recovery in photosynthesis and growth after an initial decrease in these characteristics. The seedlings grown at the highest UV-B irradiance (40% stratospheric ozone reduction) showed a more rapid increase in the concentration of UV-B-absorbing compounds and no effects of UV-B radiation on growth or phtosynthetic capacity until after four months at this irradiance. Changes in photosynthetic capacity were probably the result of direct effects on light-dependent processes, since no effects were observed on either needle chlorophyll concentrations or stomatal conductance.