Effects of simazine (2-chloro-4,6-bis(ethylamino)-s-triazine) and atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine) were observed when applied in connection with 3-amino-1,2,4-triazole (amitrole) to citrus trees. The amitrole had no significant effect. In heavy soil, atrazine toxicity to citrus trees was found during the summer following applications after a period of severe hot dry wind. The leaves became bronzed and dried, growth was checked, dry matter, chlorophyll, and catalase activity of leaves reduced, and their peroxidase activity increased when compared with controls from unsprayed plots; ascorbic acid content of leaves also was affected. Simazine, on the contrary, enhanced growth of trees, total and protein nitrogen and chlorophyll in leaves, did not affect peroxidase, catalase or ascorbic acid, and only slightly depressed dry matter content. In a light soil, probably due to less residual triazines, no toxic effects were observed and untreated controls evinced less growth, lower chlorophyll and nitrogen content of leaves, and higher peroxidase activity than all triazine treatments.