This study was undertaken to determine the effects of different chilling treatments during the dormant season upon growth of Douglas-fir seedlings0 In addition it was planned to determine whether seedlings native to various geographic areas differ in their chilling requirements0 The hypotheses were advanced, based on previous studies, that chilling at near L & 00F. was more effective in satisfying the chilling requirement of plants than chilling at temperatures near 32°F., or 50°F., and that plants native to areas of low elevation, characterized by mild winters, require less chilling than plants native to areas of high elevation characterized by long and severe winters. Verification or rejection of these hypotheses was undertaken by subjecting dormant seedlings of Douglas-fir, native to several geographic areas, to chilling under different temperature treatments for different lengths of time. Experiment I was designed to compare the effects of chilling for three, six, and twelve weeks at various temperatures (32, L & 0, 50, and 60°F., and out-of-doors) on two-year-old seedlings from eight geographic areas. After chilling, they were moved to a warm greenhouse and exposed to short days (nine hours). The effectiveness of the treatments in breaking dormancy was expressed in number of days from end of treatment to bud burst, percentage of plants which broke dormancy, and amount of shoot elongation. In general 40°F. was the most effective treatment followed by 32°F., out-of-doors, 50, and 60°F., in that order. The latter temperature had little or no effect in breaking dormancy compared with the other treatments. Twelve weeks of chilling were more effective than chilling for shorter periods at all temperatures except 60°F. At six weeks 40°F. was the most effective treatment, 32°F. was less effective, while 50, 60°F., and out-of- doors treatments were ineffective in breaking dormancy. Four weeks of chilling were ineffective regardless of the chilling treatment. Genetic differences were observed among the plants from the eight areas chilled at 40°F. for twelve weeks. Plants native to the highest areas represented in the experiment resumed growth earlier than those from the lower elevations. Experiments II and III differed from Experiment I since the dormant seedlings had germinated and grown only about three months prior to chilling. In these experiments plants were chilled for four, eight, and twelve weeks. After the chilling period half of the plants were grown under long days (normal days plus two hours of artificial light) while the other half were maintained on short days (nine hours). The results of Experiment II were similar to those of Experiment I with respect to the greater effectiveness of 40°F. compared with the other temperature8. Long days seemed to compensate for lack of adequate chilling except in plants chilled for twelve weeks at 40°F., indicating that the chilling requirements of these plants had been more adequately satisfied than by any of the other treatments. Experiment III was designed to compare the effectiveness of chilling under fluctuating day and night temperatures with that of chilling at a constant temperature. Fluctuating day and night temperatures of 40/32°F. (40°F. day, 32'F. night) were about as effective as continuous chilling at 32°F. or 40°F. Fluctuating temperatures of 50/32°F., and 50/40°F. appeared to be about as effective as continuous chilling at 50°F. Chilling at 50°F. during the day, irrespective of the temperature during night, delayed bud burst in comparison with chilling at 32, 40, or 40/32°F. Genetic differences were observed among the plants from the different areas in Experiments II and 1110 Plants native to high elevations (6000' elev.) resumed growth in lower percentages, resumed growth later, and grew less than plants from tower elevations (3000' and 3600' elev.). The principal contribution of this study has been the demonstration of differences in effectiveness of several chilling treatments in breaking the dormancy of seedlings. Variations in the chilling requirements of seedlings native to different areas has also been shown.