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Abstract. Two types of experimental evidenc2 are presented which suggest that the monoterpenes of peppermint (Mentha piperita L.) are subject to metabolic turnover. In kinetic studies with 14CO.,, peppermint cuttings rapidly incorporate label into the monoterpenes and then lose most of the label from the monoterpenes, without corresponding changes in the amount of monoterpenes present. When peppermint plants are grown in a oontrolled environment (16-hr photoperiod, 240 day, 80 night) and analyzed at intervals leaf pair by leaf pair, there is a steady increase in monoterpenes until the time of floral initiation, followed by a rapid decrease. It is suggested that monoterpenes may serve as substrates for energy metabolism in the secretory cells after other stored substrates have been depleted. In the case of monoterpenes, diurnal fluctuations in the amount of essential oil have been reported in some plants, notably in Salvia officinalis (sage) (9,18). The authors assumed that the decrease in essential oil was due to evaporation and res,inification. However, metabolic turnover is an alternate possibility. Sukhov (21) fed 14CO0 to a whole pine tree and reported that label in the monoterpenes reached a peak in 13 davs and then declined. Nicholas (15) fed 2-l4C-mevalonate to cuttings of OcimUmi basilicumn (sweet biasil) and found that radioactivity in the steam-volatile fraction reached a mlaximum after 1 hr and then decreased rapidly.We wish to report evidence for rapid metabolic turnover of monoterpenes in peppermint (Mlelthla piperita L.).