The effect of source-sink relationships on yield and its components was studied in two Tield trials. Sunflower plants (confection type, cv. DY-3) with small and large heads were treated as follows: seed from the peripheral area of the head was removed at the end of flowering, or leaves were removed soon after appearance (early leaf removal) or at the end of flowering (late leaf removal). Floret number was significantly lower in small vs. large heads, but was not affected by the source-sink manipulations. The percentage of filled seeds in the peripheral and intermediate head zones was 75-80, whereas in the central head zone it was 50-55. Neither head size nor source-sink manipulations had any effect on the percentage of filled seeds. With respect to the applied treatments, seed weight was the most responsive yield component studied. Seed weight was higher in large vs. small sunflower heads; it increased as a result of seed removal and decreased as a result of leaf removal.