Cycloheximide (CH) was applied selectively either to the shoot apex or by infiltration to the leaves of the long-day plant Hyoscyamus niger in order to investigate whether this inhibitor has an effect on the synthesis of a floral stimulus in the leaves. Treatment of the shoot apex with CH caused inhibition of the photoperiodic induction. In contrast, when CH was applied to leaves, initiation of flowering was observed under short-day conditions. The drug yielded optimum initiating effects at concentrations of 10-5--3 9 10 -~ M, inducing flowering of almost 60% of the plants. Daily infiltration over a period of up to 4 days decreased the rate of flower initiation. The effect of Ctt was shown to be additive to a photoperiodic induction, even to a sub-threshold induction, but not to 2-thi0uraeil mediated induction. In no case did the presence of additional untreated leaves on the plants suppress CH-mediated flower induction. Treatment of the leaves with chloramphenicol (10 -~ 2-10 -~ M) or puromycin (5" 10-s--2 9 10 -~ M) caused no initiating response. The results are interpreted to mean that the presence of Ctt in the leaves may lead to the synthesis of a floral stimulus also under short-day conditions. This finding is similar to that reported previously in the case of the inductive effect of 2-thiouracil. Einleitung W/~hrend durch zahlreiche Untersuchungen weitgehend gesiehert erscheint, dab der durch photoperiodische Induktion in den B1/~ttern gebildete Blfihstimulus am Vegetationspunkt fiber eine Genaktivierung wirkt, ist die Frage einer Beteiligung des Nueleins/iuresystems und einer Bildung neuer spezifischer Proteine bei der Syntbese des Blfihstimulus