The temperature and oxygen regulation of the microsomal oleate desaturase (FAD2) from safflower (Carthamus tinctorius L.) seeds was investigated. Heat-resistance profiles obtained in vivo and in vitro showed that the FAD2 enzyme maintained its maximal activity until 30 degrees C. A temperature increase from 10 to 40 degrees C caused a decrease of the FAD2 activity. However, when the temperature was decreased from 40 to 10 degrees C, no increase in the activity level was detected. The removal of hulls from safflower seeds followed by incubation in air did not change the FAD2 activity level, whereas incubation under nitrogen caused a strong decrease. Air replacement brought about the recovery of the initials levels. Oxygen concentrations less than 3% produced the inactivation of the enzyme. These data indicate that the higher thermal stability and the lower dependence on oxygen availability of the safflower FAD2 enzyme, compared with that of sunflower, could be the main factors to explain why the linoleate content of safflower seeds is more independent of growth temperature than that of sunflower seeds.