Beech mycorrhizas are capable of absorbing ammonia and simple organic compounds from solution but nitrate is a poor source of nitrogen if it is absorbed at all. The absorption of ammonia is associated with synthesis of organic nitrogen compounds in the tissues amongst which glutamine appears as the most prominent. The factors which affect absorption are those expected to affect metabolic activity. The rate of ammonia absorption is stimulated especially at certain times of year by exogenous sugars applied during or immediately before ammonia absorption. Prefeeding with either glucose or fructose stimulates absorption and so it may be concluded that trehalose, glycogen and mannitol may all be utilized in the process. Low concentrations of bicarbonate, up to 7·5 mM, enhance ammonia absorption as do succinic, fumaric and malic acids. Other organic acids applied exogenously either do not affect uptake or reduce its rate.