We report the structural analysis of the 3' noncoding region of the rat mitochondrial 3-hydroxy-3-methylglutaryl coenzyme A synthase-encoding gene. This region, encoded by a single exon of the gene, spans 410 nucleotides. By using S1-nuclease protection analysis, we have mapped two major alternative polyadenylation signals, in close proximity. Computer analysis of the 3' noncoding region has detected the existence of four inverted repeats that could form stem-and-loop structures, with the thermodynamic free energy of formation ranging from -12.2 kcal/mol to -20.4 kcal/mol. The putative regulatory role of these structural features in mRNA stability is discussed.