Horizontal gradients of ammonia have been measured in order to determine the spatial variation of the concentration above a forest as a function of distance from ammonia emitting areas. Measurements were carried out in June 1994 during conditions with winds from north and west. Concentration levels of ammonia were generally low, ranging from 0.04 to 0.7 mu g NH3-N m(-3). At northerly winds the concentration levels were about four times higher than at westerly winds, which relates well to more intensive and larger areas of ammonia emission towards north of the forest. The ammonia concentration decreases in the forest; in general the percentage decreases were lower during northerly winds (16%) than those obtained during westerly winds (67%). This is probably due to the larger extension of emitting areas towards north than towards the west. Model calculations are comparable with the measured ammonia concentration gradients, although calculated concentration levels may be higher than measured due to the use of yearly average emissions as input to the model calculations. (C) 1998 Elsevier Science Ltd. All rights reserved.