Mexican mint (Plectranthus amboinicus (Lour.) Spreng.) is a tender perennial plant in the family Lamiaceae, and used as an ornamental pot-plant in Japan. The leaves are fleshy and aromatic. This study aimed to examine the effects of light quality on the growth, leaf morphology, and essential oil production in Mexican mint. The plants propagated by cutting were transferred under respective monochromatic or combined (white) light conditions. Three monochromatic lights were irradiated with blue, green, or red LEDs which have the peak wavelength of 470, 525, or 660 nm, respectively. A combined light was simultaneously irradiated by white, blue, green, and red LEDs (the PPFD ratio was 3:1:1:1). The light treatment conditions were 24 +/- 2 degrees C, 16-h photoperiod at 100 mu mol m(-2) s(-1) PPFD. Plants were cultivated under the respective light quality conditions for 70 days. The elongation of the main and lateral shoots were significantly reduced under blue and combined lights. The growth of lateral shoots was significantly reduced under the blue light. However, the thickness and fresh weight of leaves increased under the blue light. The constitution of volatile flavors was changed by the difference of light quality. It seemed to be more active by the sesquiterpenoid biosynthesis pathway under the blue light.