1. An investigation was made into effect of daylength conditions of the inhibition content of first-year seedlings of sycamore ( Acer pseudoplatanus ). 2. The shoot apical regions and mature leaves were extracted with 80 per cent. Aqueous methanol, fractionated by paper chromatography in isopropanol/ammonia and assayed by the wheat-coleptile growth test. 3. A growth inhibitor was present in all extracts at R f 0.7. Higher levels of inhibitor were present in both apices and mature leaves of plants transferred to short-day conditions than of those maintained under long-days throughout. 4. These difference in inhibitor level can be detected after 2–5 days of short-day treatment, Preceding any marked effect of daylength on growth. 5. Evidence is adduced in support of the hypothesis that the inhibitors is produced in the leaves during darkness and is transported to the apex during the photoperiod.