Ammonia volatilization from urea-treated organic layers of four softwood and three hardwood forest stands was investigated under laboratory conditions. All samples were treated with urea at rates equivalent to 168, 336, and 500 kg N per hectare. In addition, samples from a jack pine (Pinusbanksiana) stand were treated with urea at rates equivalent to 1100 and 1680 kg N per hectare (1 ha = 104 m2). At the end of a 7-day volatilization period the pH values of L and F + H layers were determined.In both softwood and hardwood samples, the proportions of added N volatilized as ammonia were larger when higher dosages of urea were applied. It is suggested that the rate of ammonia volatilization is determined by the concentration gradient of urea in solution around the sites where the enzyme urease hydrolyses urea. Less ammonia was evolved from hardwood than from softwood organic layers. Ammonia volatilization from softwood and hardwood organic layers treated with 336 kg N per hectare in urea was 0.41 to 3.75 and 0.16 to 1.76% of the added N, respectively. At the end of the experimental period the pH values of the L layers were generally higher than those of the F + H layers. The organic layers from the hardwood stands showed a smaller shift in pH than those from the softwood stands treated with the same levels of urea nitrogen.