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The respiration rate was investigated in leaves of bearing and non-bearing spurs of biennially fruiting apple trees, Landsberger Reinette cultivar, with a Warburg apparatus. Trees in "off year" were sprayed with gibberellic acid (GA) and those in "on year" with succinic acid-2,2-dimethylhydrazide (SADH) and naphthaleneacetic acid (NAA), Two-year results showed that the respiration rate of leaves of bearing spurs was 10 to 25 per cent higher than that of the non-bearing spur leaves. GA increased the respiration of leaves of non-bearing spurs about 25 per cent, on an average. SADH and NAA greatly decreased the respiration of bearing spur leaves; the latter about 30 per cent of that of the controls. At the same time SADH and NAA caused a marked reduction of growth in terminal shoots of trees in "on year" while GA increased the growth in "off year" trees about 50 per cent. The surface of the spur leaves was inversely correlated with the respiration rates. it is suggested that the low respiration rate of spur leaves may be a condition for flower bud formation. [ABSTRACT FROM AUTHOR]