The response of RIMI [imazethapyr-tolerant (T)] sunflower (Helianthus annuus L.) hybrid and the local imazethapyr-sensitive hybrid "Zoltan" (S) was investigated under controlled conditions. Hybrids grown in pots were treated post-emergence with imazethapyr at the two pairs of leaves stage. Visual injury evaluation and vegetative parameters were recorded. ALS (acetolactate syn thase) enzyme activity was estimated in vivo 24 h after imazethapyr applica tion. Tolerance level was determined based on the resistance ratio index (ED50- herbicide dose causing 50% growth inhibition of T hybrid/ ED50 of S hybrid). Significant differences were noted between the hybrids in the in vivo ALS activ ity and vegetative parameters. Indexes of resistance ranged between 5 and 452 for vegetative parameters, whereas the index for ALS activity was 210. The data confirmed the high level of tolerance of the hybrid Rimi as compared with the hybrid Zoltan.