Under field conditions the efficacy of glufosinate-ammonium on Galium aparine L. is quite often variable. Experiments under controlled conditions with G. aparine and the more susceptible species Brassica rapa L. showed that especially low relative humidity (r.h.) and low light intensity reduced the accumulation of ammonia in the plants and glufosinate-ammonium performance. As the light intensity under field con-ditions always exceeds the intensities applied in the experiments this parameter may not be of practical importance. In contrast, low r. h. often occurs under field conditions and could explain the variable G. aparine control. A retention study showed that retention on G. aparine was significantly higher than on B. rapa but ammonia accumulation in the plants indicated that glufosinate-ammonium uptake by G. aparine was lower than that by B. rapa. Glufosinate-ammonium performance was improved on both species by adding ammonium sulphate to the spray solution; however, the increase in efficacy by am-monium sulphate was two times higher for G. aparine than for B. rapa. Ammonia accumulation in un-treated plant parts after glufosinate-ammonium application indicated some translocation of glufosinate-ammonium in B. rapa but none in G. aparine.