Concentrated urea-ammonium nitrate solution (32% N) and urea prill (46% N) were applied by helicopter at dosages of 56, 112, 224, and 448 kg N/ha before and during the growing season to four 40- to 70-year-old mixed stands of Pseudotsuga menziesii and Tsuga heterophylla. Stand growth was measured repeatedly in the next 10 years. Applying 112 kg N/ha or more to these poor-site stands increased gross and net volume growth; volume growth was related linearly to N dosage of both prill and spray. Gains from prill apparently exceeded those from spray, but a subsampling of plots indicated that actual dosages, especially of spray, were less than target dosages. At the target dosage of 224 kg N/ha, 10-year gains in gross growth averaged 34.0 m3/ha (30%) and 8.0 m3/ha (7%) after prill and spray, respectively. Although fertilization accelerated tree losses, cumulative volume of dead trees was less than 15% of gross volume growth. Season of fertilization seemed to have no effect on efficiency of either prill or spray, but suspected differences between actual and target dosages may have influenced this comparison. This concentrated N solution at dosages up to about 224 kg N/ha caused little or no increase in foliar or tip damage. Doubling this conventional dosage and applying in the growing season, however, increased visible damage and may have decreased gains in volume growth. Helicopter application of both fertilizers made this investigation resemble operational fertilization, but this increased application error and uncertainty about interpretation of results. For. Sci. 37(3):741-754.