Photosynthetic functions have been investigated in ozone stressed (200 ppb, 5 h) Melissa officinalis plants at the end of fumigation and 24 and 48 h after. Plants exhibited foliar injury and membrane permeability was significantly increased, indicating that there was membrane damage. After the end of treatment, CO 2 fixation capacity decreased and this lasted during the recovery period (until a maximum of −63% when compared to controls). These strong negative effects on photosynthetic ability were observed to be due both to stomatal and mesophyllic limitations, since stomatal conductance decreased (−23%) and intercellular CO 2 concentration significantly increased (+41%). Reduction in PSII efficiency is evidenced by (i) decrease of F v/ F 0 (−11.4%), indicating a partial inhibition at PSII donor side; (ii) significant correlation between the apparent electron transport rate through PSII and photosynthetic activity, suggesting that the O 3-induced effects are well established, as demonstrated by the development of leaf necrosis; (iii) increase in electrons required to fix one molecule of CO 2, showing a decrease in activity of photosynthetic enzymes and their ability to fix CO 2 in the presence of O 3; (iv) decrease of q L, resulting in an increase in the PSII excitation pressure. On the other hand, a regulatory adjustment of PSII efficiency was highlighted by (i) higher value of q NP, abling to counteract the negative effects of O 3 at chloroplast level because of their capacity to dissipate the excess of excitation energy; (ii) increase of the xanthophyll cycle pool size and DEPS index, showing a marked activation of photoprotective mechanisms. This represents an active response that M. officinalis initiates to cope with increased oxidative load.