Greenhouse-grown pecan [Carya illinoensis (Wangenh.) C. Koch] seedlings were treated with S as SO4-2 (0 to 4 mm) to determine the effect of S on its vegetative growth, chlorophyll concentration, nutrition, and free and protein amino acid content. Sulfur deficiency symptoms occurred when leaf S was ≈1.5 mg·g-1 dry weight or less. Chlorophyll concentration and growth increased curvilinearly with leaf S and had not peaked when leaf S was 2.7 mg·g-1 dry weight. The N : S ratio in the leaf and leaf S were equally reliable indicators of the S status of the plant, but the optimum ratio was less than the value of 15 found in other plant species. Calcium and Mg decreased with S application, suggesting a direct SO4-2 or NH4+ effect. The concentration of all other elements determined was elevated, in one or more plant parts, when S deficiency symptoms were visible. The most conspicuous effect of S deficiency on N metabolism was a greatly expanded free amino acid pool, mainly arginine. The concentration of free amino acids decreased exponentially with leaf S. Conversely, protein amino acid was inhibited by S deficiency and increased with leaf S concentration or, perhaps more specifically, with methiouine.