A specific control regulates, at the level of RNA splicing, the expression of the L1 ribosomal protein gene in Xenopus laevis. Under particular conditions, which can be summarized as an excess of free L1 protein, a precursor RNA which still contains two of the nine introns of the L1 gene accumulates. In addition to the splicing block the two intron regions undergo specific endonucleolytic cleavages which produce abortive truncated molecules. The accumulation of mature L1 RNA therefore results from the regulation of the nuclear stability of its precursor RNA. We propose that a block to splicing can permit the attack of specific intron regions by nucleases which destabilize the pre-mRNA in the nucleus. Therefore the efficiency of splicing could indirectly control the stability of the pre-mRNA.