Jonathan apple trees on MM104 rootstock, grown in Perlite in pots with controlled nutrient supply, initiated more flowers in the axillary buds of the main stem in the first year of growth if exposed for part of the time to ammonium ions than if grown on nitrate nitrogen continuously. The duration and timing of ammonium treatment was not critical; exposure for 24 h was as effective as exposure for 8 weeks and treatment as late as February was as effective as earlier treatment. Initiation was increased over most of the stem length by all ammonium treatments.