UV-A radiation (360 nm) affects the photomorphogenesis and essential-oil chemical composition of Mentha piperita. Total phenols and chlorophyll b are also affected by UV-A. When UV-A is given during the day (UV-A+W), a significant increase is found in total leaf area, leaf-area index, chlorophyll a/b ratio, total phenols, total essential-oil content, menthofuran and menthol. UV-A irradiation during the night period induces a typical shade-avoidance syndrome, with stem elongation, decreased leaf area and leaf-area index, decreased protein and total phenol content, and decreased essential-oil and menthol content. The possible interference of UV-A with phytochrome is discussed.