Glutathione S-transferase (GST) activity pattern was determined in tolerant (cv. Sumai#3) and susceptible (cv. Falat) wheat heads inoculated with Fusarium graminearum, the causal agent of head scab disease (FHB), during various developmental stages. GST specific activity exhibited a transient pattern in Sumai#3 reaching a maximum level at the milk stage and declining thereafter. GST level in Sumai#3 was three times greater than that in the susceptible cultivar. Thin layer chromatography and densitometry were used to measure zearalenone (ZEN) content in FHB-inoculated wheat heads. Zearalenone concentration reached a maximum level in Sumai#3 at the milk stage and thereafter it declined. ZEN content in the susceptible wheat heads continued to increase and reached a maximum level at the ripening stage. There seems to be a correlation between the increased GST activity and significantly lower ZEN production in the tolerant wheat heads.