The application of gibberellin to Acer negundo either during or after a short photoperiod strikingly lowered the amount of hardiness obtained after 4 weeks in darkness at 5 degrees . Two growth retardants, B9 and Amo 1618, the latter of which interferes with gibberellin synthesis, brought about hardiness increases under long photoperiods. The naturally occurring inhibitor, dormin, also increased hardiness under the usual inhibiting influence of long photo-periods. Extracts from plants given long or short days had gibberellin-like compounds in largest quantities during LD and lowest quantities under SD, while the inverse was true for the inhibitor.