The role of nitrogenous compounds in the pest‐host interaction ofAradus cinnamomeusandPinus sylvestriswas studied by analysing free amino acids and related compounds in the insects and the cambial layer of Scots pine. About 35 different compounds were identified by means of thin‐layer and automatic ion‐exchange chromatography. The total content of amino acids in the insects was ca. 0.86% of the fresh weight (65 μmoles/g), ca. 0.075% (7.2 μmoles/g) in the young phloem layer of Scots pine, and ca. 0.078% (6.8 μmoles/g) in the young xylem layer. The most abundant free amino acids in the insects were proline and glutamine, followed by alanine, glutamic acid, taurine, histidine, arginine, and lysine. The amino acid composition was similar in males and females, and almost identical in brachypterous and macropterous females. The concentration of glutamic acid was low in the hemolymph but high in the salivary glands and other tissues. In other respects the amino acid composition of the salivary glands resembled that of the whole insect. No distinctly phytotoxic compounds were identified in the insects. The predominant amino compounds in the cambial layer of the pine saplings were γ‐aminobutyric acid, glutamine, ethanolamine, serine and alanine. The amino acid composition was similar in extracts of phloem and xylem. The bulk of the alcohol‐soluble compounds in the pine cambial layer were carbohydrates (ca. 3.5% of fresh weight).