Abstract The study examined the influence of light and auxin on the transcription level of PnACO3, a gene involved in ethylene production, in relation to the inhibitory effect of ethylene on flower induction in the short-day plant Pharbitis nil (=Ipomoea nil). Exogenous auxin was shown to increase the level of PnACO3 mRNA, with the effect depending on the experimental conditions. Light did not affect the level of PnACO3 mRNA. Applying auxin to seedling cotyledons at the beginning of inductive night boosted PnACO3 transcriptional activity even threefold during the next few hours, supporting our previous suggestion that the inhibitory effect of auxin on P. nil flowering results from its stimulatory effect on ethylene production.