Peppermint (Mentha piperita L.) was grown in a growth chamber under several combinations of temperature and illumination, and the monoterpenes of each leaf pair were analyzed by gas chromatography. Effects on the monoterpenes could be seen in the new leaves after a few days in the growth chamber. Long-day conditions enhanced growth, with a corresponding increase in the total amount of monoterpenes. Either short nights or cool nights, combined with full light intensity during the day, enhanced the formation of menthone and depressed the accumulation of menthofuran and pulegone. Experiments with interrupted night and with low light intensity indicated that photoperiod, as such, does not directly influence the terpene composition. It is suggested that the oxidation-reduction level of the monoterpenes reflects the oxidation-reduction state of the respiratory coenzymes of the terpene-producing cells, and that this, in turn, depends on the concentrations of respiratory substrates in the cells. This suggestion is based on the likelihood that warm nights cause depletion of respiratory substrates, resulting in oxidizing conditions, while cool nights preserve high levels of respiratory substrates, and thus maintain reducing conditions.