The areAr-18 mutation is a loss-of-function mutation in areA, the positive acting regulatory gene mediating nitrogen metabolite repression in Aspergillus nidulans. It results from a reciprocal translocation which splits the coding region into 5' and 3' moieties. Surprisingly, we have selected rare intracistronic revertants of areAr-18. From crosses heterozygous for areAr-18 revertant alleles, duplication-deficiency progeny containing two copies of a substantial portion of chromosome IV but lacking part of chromosome III, including the 5' moiety of areA, have been obtained. For all four revertants analysed genetically, growth properties of these duplication-deficiency strains indicate that the reversion events involve the 3' portion of areA and that the 5' portion of areA is unnecessary for the revertant phenotype. This conclusion was directly confirmed for one revertant using Southern blotting. As all four reversion events involve additional chromosomal rearrangements, they probably fuse functional promoters, ribosome binding sites and 'in frame' initiation codons to the 3' portion of the gene. In the course of characterisation of these mutations, new mapping data for a large region of chromosome IV have been generated, and a new reciprocal translocation activating the cryptic regulatory gene areB, whose product can substitute for that of areA, has been identified.