Manganese (Mn) toxicity may be a significant constraint to forest health on acidic, non-glaciated soils. We hypothesised that sugar maple (Acer saccharum Marsh.) and red maple (Acer rubrum L.) seedlings differ in their tolerance to excess Mn, and that photosynthetic sensitivity to excess Mn is exacerbated at higher light intensities through photo-oxidative stress. To test these hypotheses, we assessed photosynthesis and antioxidant enzyme responses of sugar maple and red maple seedlings at variable Mn and light levels in a greenhouse study. In both species, high Mn treatments impaired photosynthetic function, particularly in high light conditions. Responses to Mn and light depended on the developmental stage of the leaves. All sugar maple leaves were sensitive to Mn toxicity except shaded young leaves. For red maple, only mature leaves exposed to high light were prone to Mn toxicity. Antioxidant enzyme and Ф_PSII / Ф_CO2 data suggested that photo-oxidative stress did not explain the observed photosynthetic responses to treatment variables. Our results indicate that in natural forest environments, sugar maple and red maple foliage exposed to high light intensity (outer canopy, canopy gaps) may be more prone to Mn toxicity.