o look for the possible influence of photosynthesis and photorespiration on terpene metabolism in peppermint (Mentha piperita L.), the volatile oil composition in the peltate glandular trichomes was estimated in relation to: 1, 5-Ribulose bis-phosphate carboxylase (RuBP-case)and glycolate oxidase (GO)activities. Total protein and chlorophyll content and chorophyll a/b ratio values were also estimated and correlated with the oil composition. The chloroplast ultrastructure, some chloroplast morphometrical parameters and the photorespiratory organelle complex (mitochondrion-microbody-chloro-plast)ultrastructure were also studied. The effect of 3-3, 4(Dichlorophenyl)-1,1-dimetlrylurea (DCMU)and 1, 1'-Dimethy 1-4, 41-bipyridinium dichloride (Paraquat)on oil composition was tested on peppermint leaf discs. The percentage of monoterpene alcohols and esters increased, along with RuBP-case and GO activities, with leaf age, while the percentage of monoterpene ketones and the Chl a/b ratio decreased. The decline in the Chl a/b ratio and the increased number of thylakoids per granum in the chloroplasts suggested a relative increase in Photosystem II (PSII)activity (compared to PSI)and thus an increase in NADPH2 levels with leaf development. Photosynthetic electron transport inhibition made by DCMU and Paraquat on peppermint leaf discs caused a decline in the content of monoterpene alcohols suggesting that the photosynthetic NADPH2 production may be at least partially responsible for the progressive transformation of monoterpene ketones to alcohols. Increased rates of carbon availability were correlated with oil synthesis only before the full leaf expansion. © 1990 Taylor & Francis Group, LLC