Mesotrione, a carotenoid biosynthesis inhibitor, is currently being evaluated for its use in turfgrass. Mesotrione has been reported to injure perennial ryegrass (Lolium perenne L.) in certain environmental conditions. Variation in control due to environmental conditions has been reported in other species. Therefore, research was conducted to investigate the effects of light intensity and temperature on perennial ryegrass carotenoid composition following mesotrione application. Perennial ryegrass was treated with mesotrione (0.28 kg ai/ha) and subsequently placed at relative high, medium, or low light intensity at 18, 26, or 34 ºC with a 16 hour photoperiod. Leaf tissue was harvested 0, 3, 7, and 21 days after treatment (DAT). Percent bleaching, percent necrosis, foliar-weight and F_V/F_M were recorded as an indication of mesotrione efficacy. Carotenoids were quantified using HPLC analysis. Various carotenoids decreased following mesotrione treatment. Effects of temperature and photoperiod were less dramatic.