Safflower (Carthamus tinctorius L. cv. Gila) was grown in solution culture; the roots were inoculated with zoospores of Phytophthora cryptogea 28 d after sowing. The period for which roots were subjected to hypoxia prior to inoculation (5%(v/v) O2 in the gas stream) had a profound influence on the degree of hyphal damage to roots 8 d after inoculation. Roots exposed to hypoxia for 7 d prior to inoculation were not more than 20% necrotic 8 d after inoculation and the shoots were fully hydrated, presumably through sustained water transport by the root. Plants which were exposed to hypoxia for just 1 d after inoculation, on the other hand, developed almost total necrosis of the root system and the shoots wilted severely subsequent to infection. We propose that while short periods of hypoxia pre-dispose safflower roots to hyphal infection, a longer period of adaptation to hypoxia reverses this susceptibility. The mechanism for this protective effect, while not known, could reside in root aerenchyma formation, phytoalexin synthesis, or other metabolic and ultrastructual changes characteristic of roots exposed to low O2 conditions.