In maize chloroplasts, the ratio of HCO(3) (-) (anion) binding sites to high-affinity atrazine binding sites is unity. In the dark, atrazine noncompetitively inhibits the binding of half of the HCO(3) (-) to the photosystem II (PSII) complexes. The inhibition of binding saturates at 5 micromolar atrazine, little inhibition is seen at 0.5 micromolar atrazine, although the high-affinity herbicide binding sites are nearly filled at this concentration. This means that HCO(3) (-) and atrazine interact noncompetitively at a specific low-affinity herbicide binding site that exists on a portion of the PSII complexes. Light abolishes the inhibitory effects of atrazine on HCO(3) (-) binding. Based on the assumption that there is one high-affinity atrazine binding site per PSII complex, we conclude that there is also only one binding site for HCO(3) (-) with a dissociation constant near 80 micromolar. The location of the HCO(3) (-) binding site, and the low-affinity atrazine binding site, is not known.