Two-chloro-4-ethylamino-6-isopropylamino-s-triazine (atrazine), 5-bromo-3-sec-butyl-6-methyluracil (bromacil), and 6,7-dihydrodipyrido-(1,2-a:2’,1′-c)-pyrazidiinium salt (diquat) significantly inhibited photosynthetic C14O2-fixation to varying degrees in corn (Zea mays L., var. Dixie 18), cotton (Gossypium hirsutum L., var. Deltapine), and soybeans (Glycine max (L.) Merr., var. Lee). Atrazine treatments of 10 ppm overloaded the protective mechanisms in corn and cotton. Both 1 and 10 ppm of atrazine reduced C14O2-fixation to less than 5% of controls in soybeans, a sensitive species. One ppm of bromacil and of diquat reduced C14O2-fixation in all three crops, but diquat was less inhibitory than either bromacil or atrazine. Except for diquat on corn, herbicides had no significant effect on C14O2-fixation in the dark. Treatments in which photosynthetic C14O2-fixation was 5% or more of controls had little effect on the kinds or relative amounts of C14-labeled compounds produced. Only the total amount fixed was reduced. Treatments, in which photosynthetic C14O2-fixation was less than 5% of controls, caused a significant reduction in the relative amounts of sucrose and alanine produced; however, the relative amounts of malic, aspartic, and glutamic acids increased.