Several photosynthetic inhibitors were screened for relative photosynthetic reduction on leaves of greenhouse-grown peach trees. The most effective were 8% Vapor Gard, 10M diurcn, 10⁻³M ABA, and 10⁻³M terbacil, all of which reduced net photosynthesis (Pn) rates by 50% or more by 1 day after treatment without causing excessive phytotoxicity. Of these 4 treatments, terbacil at 10⁻³M gave the most consistent effects; treated leaves re-established near normal Pn rates within a week of treatment. At 1, 3, and 7 days after treatment with terbacil at 10⁻³M, Pn rates were 1.1, 9.0, and 16.6 mgCO₂ dm⁻² hr⁻¹ , respectively, compared with a pretreatment rate of 19.7. Terbacil was applied as limb treatments to bearing peach trees in the field to determine fruit thinning activity. Treatments were: single applications of 1000 or 2000 ppm, double applications a week apart of 1000 ppm, or 2000 followed by 1000 ppm. All treatments significantly reduced fruit number. Fruit drop was 73 to 90% on treated limbs compared to 20% on non-thinned check limbs. Only the single 1000 ppm treatment resulted in fruit size similar to the hand-thinned check, however, all treatments increased fruit size over the non-thinned checks. / M.S.