We have studied the inhibitory effect of heterocyclic herbicides simazine. paraquat, pyrazon and amitrole on photosynthetic CO2 fixation and on the level of intermediates of the CO2 assimilation cycle in isolated spinach (Spinacia oieracea) chloroplasts, as well as their in vitro activities on ribulose-l,5-bisphosphate carboxylase and fructose-1,6-bisphosphatase. The half inhibitory concentrations (Is,,) of CO2 assimilation were about 1 \LM for simazine and paraquat, and 10 \iM for pyrazon. Amitrole, with an Is,! 100 |i.M. gave only a weak inhibition. In the presence of simazine or pyrazon the triose-phosphates/phosphoglycerate ratio diminished because of a decrease of the triose-phosphates percentage from 47% to 19%, which means an inhibition of the phosphoglycerate reduction step by a low NADPH synthesis. However, there was not a parallel increase of phosphoglycerate, because of collateral pathways leading to phospho-enolpyruvate, amino acids and other non-identified compounds. Paraquat did not give such a decreased ratio, which could be explained as an inhibition of some step of the Calvin cycle later than triosephosphates by the H^O^ formed in a Mehler reaction. Amitrole did not show any effect on the pattern of intermediates. Simazine and pyrazon at 10 [iM concentration promote a 20-30% activation of ribulose-l,5-bisphosphate carboxylase activity, whereas paraquat, pyrazon and simazine showed an I,,, about 100 \iM for the inhibition of the photosynthetic fructose-1,6-bisphosphatase.

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