Changes in essential oil content, CO 2 exchange rate and distribution of photosynthetically fixed 14CO 2 into essential oil, amino acids, organic acids and sugars were determined in developing peppermint leaves. The incorporation of 14CO 2 into sugars was maximal followed by organic acids, amino acids and essential oil at all stages of leaf development. The incorporation into sugars and amino acids declined as the leaf matured whereas that in essential oil and organic acids increased with leaf expansion and then decreased.