The effects of the new growth retardant tetcyclacis (TCY) on stem growth and endogenous gibberellin (GA) levels were investigated in the long-day rosette plant Agrostemma githago. Application of TCY (10 ml of a 5·10(-5)M solution daily) to the soil suppressed stem elongation in Agrostemma grown under long-day conditions. A total of 10 μg GA1 (1 μg applied on alternate days) per plant overcame the growth retardation caused by TCY.Control plants and plants treated with TCY were analyzed for endogenous GAs after exposure to nine long days. The acidic extracts were fractionated by high-performance liquid chromatography. Part of each fraction was tested in the d-5 maize bioassay, while the remainder was analyzed by combined gas chromatography-selected ion monitoring. The bioassay results indicated that the GA content of plants treated with TCY was much lower than that of untreated plants. The data obtained by gas chromatography-selected ion monitoring confirmed that the levels of seven GAs present in Agrostemma were much reduced in TCY-treated plants when compared with the levels in control plants: GA53 (13%), GA44 (0%), GA19 (1%), GA17 (33%), GA20 (15%), GA1 (4%), and epi-GA1 (13%). These results provide evidence that TCY inhibits stem growth in Agrostemma by blocking GA biosynthesis and thus lowering the levels of endogenous GAs.