Black oak (Quercus velutina Lam.) and gray birch (Betula populifolia Marsh.) trees were defoliated in 0, 1, 2, or 3 successive years. Concentrations of 8 minerals, 4 sugars, and 25 amino acids in the foliage of these trees were measured when gypsy moth, Lymantria dispar (L.), reared on them were in instars I, III, IV, and V. These concentrations were tested for changes among years, and changes due to previous-and current-year defoliations. Most foliar constituents varied in concentration from year to year, though relatively few were affected by current or previous defoliations. In black oak, concentration of total free sugar measured during the fifth instar was reduced by current defoliation and correlated with gypsy moth pupal weight. In gray birch no decrease in sugar concentration due to defoliation was apparent, but pupal weights of gypsy moths reared on these trees were correlated with the ratio of total free sugar to calcium in the foliage measured during the fifth instar. Some implications of these apparent relations for gypsy moth larval growth and population dynamics are discussed.