This research addressed the effects of simulated acid rain on the chlorophyll contents of leaves of Pinus densiflora, on the net photosynthetic rate on a unit chlorophyll basis, and on chlorophyll degradation. When NO3− concentrations in rain water increased, the chlorophylla+b contents of the leaves increased in parallel, even though the pH of the simulated rain decreased. The nitrogen content of the rain was sufficient to stimulate the production of chlorophyll and was able to override any degradative effect expected due to the H+ input from acid rain. The absorption spectrum of chlorophyll extracted from the leaves exposed to acid rain showed peaks at 665, 606 and 535 nm, characteristic of pheophytin a, indicating degradation of chlorophyll to pheophytin. Net photosynthetic rate on the basis of chlorophyll content was reduced by acid rain, even though the total chlorophyll content of the leaves was increased. The acid rain does, therefore, simultaneously increase chlorophyll content and reduce the efficiency of the use of chlorophyll in photosynthesis. This reduced efficiency may be linked to the increase in the rate of degradation of chlorophyll to pheophytin.