The effects of elevation (200, 950 and 1760 m) and season (April-October) on leaf morphological, anatomical, ultrastructural, morphometrical and photosynthetic parameters were studied in Origanum vulgare plants. Observations aimed at the determination of the alterations in leaf structure and function associated with differential growth and adaptation of plants. Raising elevation results in a progressive decrease of plant height. During the growing period, summer plants are taller than spring and autumn plants at all elevations examined. In high-altitude populations (O. vulgare ssp. vulgare), the blade size becomes reduced in June leaves as compared with October leaves, while it does not change remarkably in low-altitude populations (O. vulgare ssp. hirtum). Leaf thickness remains more or less stable during the growing period. Expanded leaves in June and October at 200 m elevation contain dark phenolics only in their epidermis, whereas leaves of August are densely filled with phenolics in all of their tissues. In June at 1760 m elevation, leaves are devoid of phenolics, which, however, occur in the epidermis of the leaves in August and October. At higher altitudes, larger mesophyll chloroplasts with more starch grains are present in June leaves, whereas in August and October leaves chloroplasts are smaller with fewer starch grains. Leaf stomata and non-glandular hairs increase in number from the lowland to the upland habitats, whereas glandular hairs decrease in number. During the growing season, the density of stomata and of glandular and non-glandular hairs progressively increases. In the low- and mid-altitude oregano populations, leaf chlorophyll a content and PSII activity significantly increase in October, whereas they simultaneously decrease in the high-altitude population, suggesting a phenomenon of chilling-induced photoinhibition. The highest photochemical efficiency of PSII appears in the mid-altitude population (having characteristics intermediate between those of O. vulgare ssp. hirtum and ssp. vulgare) where environmental conditions are more favourable. This conclusion is also confirmed by the observation that the 950 m O. vulgare population has larger and thicker leaves with highly developed palisade and spongy parenchymas.