The nucleotide composition of whole cell RNA from Epilobium leaves at different developmental stages was determined during various growing periods. Differences in the nucleotide composition were found to be correlated with the RNA content of the leaves and with the development of the plants. An expected correlation of the base-composition of RNA with the extrachromosomal genetic constitution of the investigated plants has not been demonstrated because of the different physiological behaviour of these plants. An RNA-fraction, not hydrolysable with 0,3 N KOH proved to be partially responsible for the observed differences in the RNA-nucleotide composition.