The uptake of mercury vapor by six gramineous plant species was compared under uniform conditions using a whole-plant chamber and 2"Hg-labeled mercury at a low atmospheric concentration. Mean Hg uptake by leaves of the C3 species oats (Avena sativa), barley (Hordeum vulgare), and wheat (Triicum aestivun) was 5 times greater than that by leaves of the C4 species corn (Zea mays), sorghum (Sorghm bicolor), and crabgrass (Digitaria sangualis). Mthough there was a difference in resistances associated with vapor entry into the leaves, as shown by estimates of gas exchange, the differential uptake by C3 and C4 species was largely attributable to internal resistankces to Hg vapor binding. The nature of the internal resistances and the site or sites of Hg vapor binding remain unspecified.In an earlier paper (3), we reported the results of a study into the uptake, by wheat, of metallic mercury vapor, the principal form of atmospheric mercury pollution. Essentially all Hg accumulation was found to be confined to the leaves, and a simple gas exchange model was employed to elucidate the effect of several environmental parameters upon the rate of Hg vapor uptake. The model was of the form,