Solanum tuberosum L. cv Norchip plants were grown in open-top chambers in the summer of 1986. Plants were treated with charcoal-filtered air, nonfiltered air, or nonfiltered air supplemented with 33, 66, or 99% of the ambient ozone (O 3) concentrations from 1000 to 2000 h eastern daylight time daily. In addition, plants received charcoal-filtered air plus 0, 0·15 (393 μg m −3), 0·34 (891 μg m −3), or 0·61 (1598 μg m −3) ppm sulfur dioxide (SO 2) from 0900 to 1200 h once every 14 d for a total of four treatments. Ozone induced a linear reduction in number and weight of Grade One (> 6·35-cm diameter) potato tubers and in total weight of tubers. Ozone also induced linear reductions in the percentage of dry matter of tubers and linear decreases in glucose and fructose content of Grade One tubers. Sulfur dioxide induced a stimulation and then decline of the number, percentage of dry matter, and sucrose content of Grade One tubers. The SO 2 response best fit a quadratic curve. No O 3 × SO 2 interactions were detected for any of the yield or quality functions measured.