Simazine and dalapon have been used to control weeds in many crops during the last few years, but in 1963, when this study was begun, their uses in fruit orchards were limited. To study the effects of simazine and dalapon both on the control of weeds and on various metabolic changes in apple (Malis pumila), Montmorency sour cherry (Prunus cerasus), sweet cherry (Prunus avium), and Prunus mahaleb root s tock, experiment s were conducted from 1964 through 1966 in both the field and in the greenhouse. The application of a low do sage of simazine (4 lb/A) to soil around twelve-year-old Montmorency sour cherry trees in the field resulted in 60 to 70 percent weed control, while tree growth, chlorophyll content of the leaves, fruit y i e ld, and fruit quality were enhanced. Simazine applied at the higher dosages (8 and 12 lb/A) gave very good weed control (up to 100 percent) but reduced tree growth, chlorophyll content, and fruit yield and quality. In the greenhouse , the application of simazine at all three levels caused severe damage to R· mahal eb and sweet cherry trees and hence reduced the water uptake , but young apple trees treated with simazine showed no visible damage , although water uptake was reduced. In young sour cherry trees treated with 4 lb/A simazine, slight chlorophyll increase s we re observed , while at the higher dosages , decreases were observed. Simazine applied to the soil surface was absorbed readily by all trees. It accumulated in large amounts in the l eaves compared with the other parts of these plants. Sour cherry trees accumulated more simazine than apple trees. Results from both field and greenhouse trial sindicate that apple trees were comparatively resistant, and sweet cherry and ~ · mahaleb trees were very sensitive. Enzyme analys i s indicated that simazine caused an increase in the activity of peroxidase and polyphenol oxidase in all trees. The activity of catalase and cytochrome oxidase was decreased in f. mahaleb, sweet cherry , and sour cherry trees, but apple trees showed an increase in both catalase and cytochrome oxidase activity. Compared to the other trees , the most tolerant , untreated trees (apple) had relatively high peroxidase and low polyphenol oxidase activity, with moderate catalase and cytochrome oxidase activity . The moderately resistant species ( sour cherry) had moderate peroxidase, catalase, and cytochrome oxidase activity but very low polyphenol oxidase activity , while the sensitive trees (~ . mahaleb and sweet cherry) showed a wide range of enzyme activities . ~ · mahaleb leaves had low peroxidase and catalase activities and high polyphenol oxidase and cytochrome oxidase activities, but sweet cherry leaves had moderate peroxidase and polyphenol oxidase , high catalase, and l ow cytochrome oxidase activities. The movement of simazine in the soil was s low. Regardless of applied amounts, most of the herbicide stayed in the top 0-6 inches of soil after surface application, but movement in t he soil increased as the rate of application increased. The post-emergence application of dalapon to a Montmorency sour cherry orchard reduced weed growth in general, but complete weed control was not observed. Tree growth, chlorophyll content of the l eaves, fruit yield, and fruit quality of dalapon-treated Montmorency sour cherry trees were reduced compared with the untreated control. These reductions became greater as t he dalapon dosage increased .