Birch seedlings ceased extension growth when transferred from long days (LD) to short days (SD). The cessation of growth under SD was associated with enhanced rates of photosynthesis, probably due to a lowered diffusive resistance to gas exchange. Endogenous levels of ABA were lowered under SD but the degradation of 2-¹⁴C(±)ABA was not disturbed. It is concluded that the photoperiodic control of extension growth in Betula lutea is not mediated by ABA, although hormonal changes following transfer to SD may find some expression in an altered stomatal physiology.