In the pot experiment, conducted in a phytotron at a constant temperature 22°C and artificial lighting, the effectiveness of growth regulator concentrations on the inhibition of shoot growth of perennial ryegrass (Lolium perenne L.) and cocksfoot (Dactylis glomerata L.) was evaluated. In the study, a single growth inhibitor, with the trade name Flordimex SL 420 (active substance: ethephon, 2-chloroethylphosphonic acid), was applied in varying concentrations of 10%, 20% and 30%. In the experimental series, every 3 days in the period from 7 May to 6 June 2008, 10 measurements were performed. Empirical data were statistically evaluated. In the experiment with perennial ryegrass and cocksfoot, it was found that 30% of inhibitor concentration used inhibited the growth of shoots in the grass species studied.