In a 14-week study, 1-year-old Aleppo pine seedlings were grown in two growth chambers. Seedlings were artificially hardened by decreasing photoperiod and temperature. In each chamber half of the seedlings were fertilized with nitrogen (8.4 mg seedling(-1)). In order to determine the relative importance of the hardening environment versus fertilization, each chamber was programmed to decrease night temperatures down to a low of 8 or 4 degrees C. Chlorophyll fluorescence and frost hardiness was measured five times during the experiment. A sample of seedlings from each treatment was exposed to an artificial frost at -5 degrees C and the freezing effects were assessed by measurements of chlorophyll fluorescence and visual evaluation of needle damage. Seedlings increased their frost hardiness during the experiment in all the treatments but the ratio of variable to maximal chlorophyll fluorescence (F(v)/F(m)) measured before freezing did not vary during the experiment. This indicates that Aleppo pine maintains its photosynthetic ability during hardening in contrast to other coniferous species from colder climates. The effect of nitrogen fertilization on frost hardiness was small in comparison with chamber effect. Nitrogen fertilization slightly delayed the acquisition of hardening in the coldest chamber. Seedlings in the warmest chamber did not become fully resistant to -5 degrees C, but in the coldest chamber, where night temperature reached 4 degrees C, all the seedlings were resistant to the frost. Severe damage caused by frost could be related to a rapid rise of minimal fluorescence (F(0)) but the best index of damage was the drop of F(v)/F(m) after freezing.