Atrazine (2-chloro-4, ethylamino-6-isopropylaminos- triazine) applied to Hordium vulgare L. variety barbless in low concentrations (0.001, 0.01, 0.1 uM) was shown to in--­ crease the activity of D-ribulose-1, 5-diphosphate carboxylase (RuDPCase) and phosphoribulokinase (PRuK) up to 23% and 30% respectively and increase the amount of chlorophyll and extractable protein up to 18% and 21% respectively over the control plants. These results indicate that sub-toxic levels of atrazine significantly stimulate plant growth. Barley plants given higher concentrations of atrazine (1.0 and 10 uM) showed up to a 78% and 42% decrease in the activity of RuDPCase and PRuK respectively as well as a decrease in the amount of chlorophyll and extractable protein--72% and 19% respectively as compared to control plants. Zea mays L. variety PX 545 plants treated with the higher concentrations of atrazine (1.0 and 10.0 uM) showed no deleterious effects of the herbicide and in fact showed some enhancement of growth. Treatment of corn with the lower concentrations of atrazine (0.001, 0.01, and 0.1 uM) resulted in little or no significant differences from the control plants.