The mesophyll ultrastructure of needles of Norway spruce [Picea abies (L.) Karst.] and Scots pine (Pinus sylvestris L.) seedlings was examined in the course of experimental acid rain treatment lasting three years. The pH 3-treated needle mesophyll cells exhibited thicker granal packing in their chloroplasts than the dry control needles after one season's irrigation. Starch occurrence had decreased due to pH treatment when the whole hardening and rest period was considered. Chloroplast shapes were seen to vary from ellipsoid to irregular, especially in the pH 4 and pH 3-treated needles, due to divisions during hardening. The mean area of mesophyll chloroplasts in the hardened state after three seasons, irrigation was largest (10.3 μm2) in the pH 3-treated needles, which also contained the smallest number of dark-stained plastoglobuli. Starch occurrence in the hardened mesophyll cells in October reflected disturbances in the hardening processes in the pH 3-treated needles.