Flowering is a crucial turning point in the life cycle of most plants. The process of flowering is controlled by external factors such as light and temperature. Floral induction is the first step in the transition from the vegetative to reproductive stage of development. In the photoperiodically sensitive plants this process is regulated by the duration of light and darkness during a 24-h cycle. The aim of our study was to determine whether the undifferentiated callus tissue obtained from cotyledons, is suitable for molecular investigations on the mechanisms of flower induction. The callus tissue was obtained from cotyledons of Pharbitis nil plants, which were cultivated in inductive or non-inductive conditions. The callus obtained after two subcultures was used for isolation of RNA. The total RNA was extracted as described by Chomczynski (1993). We have examined the changes in the pattern of RNA in these two types of callus, using the technique of differential display by the polymerase chain reaction (PCR). Differential display is a method for the identification and cloning of differentially expressed eucaryotic genes. We have found the differences between patterns of RNA derived from callus tissue cultivated under non-inductive conditions and callus tissue cultivated under inductive conditions. In conclusion we can suggest that the tested callus preserved the information on the photoperiodic induction in cells. The process of undifferentiation did not result in the loss of the properties acquired by cotyledon tissue during the photoperiodic treatment.