Zea mays L., cv. Potro, shoots had a higher level o f resistance to imazaquin (IQ) and metsul furon methyl (M SM ) than roots. Shoot lengths were increased by pretreating the seeds with commercial 1,8 -naphthalic anhydride (N A ) at 1% (w/w) or oxabetrinil at 0.2% (w/w). The growth o f shoots o f safened seeds was unaffected by 400 nM IQ and by 40 nM MSM. The in vivo activity o f acetolactate synthase (ALS) extracted from corn shoots and roots was not affected by treatments with IQ or MSM, but pretreatment o f seeds with N A or oxabetrinil, prior to germination, caused an increase in the level o f extractable ALS from shoots. ALS activity from roots and shoots o f NA-pretreated seedlings was increased to a large degree (> 40% ) when the seedlings were germinated on 40 nM MSM, whereas ALS acti vities from oxabetrinil-pretreated seedlings were enhanced to a lesser degree (—20%). ALS from unsafened seedlings was inhibited 21% by 400 nM IQ and 70% by 40 nM MSM in vitro, but ALS from roots o f seedlings germinated on 400 nM IQ was not inhibited by 400 nM IQ in vitro.