6-benzylaminopurine (BA), which was applied at the 10 mm stage of fruit development at 50 and 100 mg.liter$\sp{-1}$ to 'McIntosh' apples (Malus domestica Borkh), effectively thinned fruit, and increased fruit size and caused seed abortion. Regardless of BA treatment, there were two distinguishable peaks of fruit abscission during 'June drop' on 'Morespur McIntosh' apples. There was a large peak about 14 days after treatment and a second smaller one 24 days after treatment. The first abscission peak was increased by BA, and approximately half of then existing fruit abscised from treated trees between 10 and 14 days after treatment, whereas only one fourth abscised from the control trees. The number of seeds per fruit was lower in abscising fruit than in persisting fruit regardless of BA treatment. Seed number of abscising fruit was increased linearly by BA application. BA enhanced the translocation of $\sp{14}$C-sorbitol from leaves to fruit when applied directly to the fruit, but not when applied directly to the leaves. Therefore, it is unlikely that BA thinned apple fruit directly through restricting the translocation of photosynthates moving from leaves to fruit. Net photosynthesis was decreased and dark respiration was increased when temperature following BA application was high (30$\sp\circ$C), whereas there was no effect when temperature was lower (20$\sp\circ$C). Total nonstructural carbohydrates, total soluble sugars, reducing sugars, and starch in the leaves decreased dramatically over the 12 or 13 day observation period, regardless of BA treatment. These carbohydrate levels were further lowered by BA application. Abscising fruit, based on specific reddening of pedicel, had higher reducing sugars, total soluble sugars, starch, and total nonstructural carbohydrates. The levels of these carbohydrates in fruit from BA treated trees were similar to that from untreated control trees. Foliar application of 5% sorbitol solution had no effect on fruit set. Removal of bourse shoot tip increased fruit set regardless of BA treatment. BA thinned fruit regardless of whether shoot tips were removed or not, and there was no interaction between BA application and shoot tipping. Experiments were conducted to evaluate the effects of BA on fruit set and fruit development on small fruiting branches supported by different numbers of leaves per fruit. BA thinned fruit only when one leaf per fruit was on the girdled small fruiting branch, but not when the leaf number was greater than two. Fruit weight and soluble solids content increased quadratically with increasing leaf number per fruit. BA reduced fruit growth rate when less than 16 leaves were present on the girdled branches between 3 and 7 days after treatment, but it did not affect fruit growth rate when 32 leaves were on the girdled branches. Increasing leaf number also increased normal seed number while decreasing the number of aborted seeds, but it had no effect on the number of total seeds per fruit. BA reduced the number of normal seeds per fruit only when the number of leaves per fruit was less than 4. These results were discussed in relation to carbohydrate supply and source-sink relationships.