Leaf tissue of diploid, triploid, and tetraploid tomatoes (var. UF-36) and diploid and tetraploid barley (var. Steel and Midsummer) were analyzed for total chlorophyll, soluble protein, dry weight, ribulose-1,5-diphosphate carboxylase, phosphoribulokinase, and peroxidase. Polyploidy caused a slight increase of all factors in Midsummer rye, had a variable effect on Steel rye and Everest barley, and caused a reduction in tomatoes and Barbless barley when the factors were determined on a fresh weight basis. In tomatoes, if the ribulose-1,5-diphosphate carboxylase activity was determined on protein content, dry weight or cell number the tetraploid form had higher activity than the diploid form. The interaction between polyploidy and the leaf tissue changed with the time of leaf initiation and also with the age of the leaf. The specific activity of ribulose-1,5-diphosphate carboxylase did not change with changes in chromosome number. However, there was a large drop in the specific activity in all plants assayed between the middle of October and the middle of November.