We have exploited a modular cat reporter system (Vega Laso, M. R., Zhu, D., Sagliocco, F. A., Brown, A. J. P., Tuite, M. F., and McCarthy, J. E. G. (1993) J. Biol. Chem. 268, 6453-6462) to investigate the relationship between mRNA structure, translation, and stability in the yeast Saccharomyces cerevisiae. The stability of the cat mRNA was not influenced by changes in the length and nucleotide sequence of the 5'-leader, but was affected by the formation of stable 5'-secondary structures (> -15 kcal.mol-1). Cat mRNA stability changed only slightly when the CYC1 3'-trailer was replaced with PGK1 sequences, and was influenced by some secondary structures in the 3'-trailer. Secondary structures formed by interactions between the 5'-leader and 3'-trailer increased the stability of the cat mRNA. However, all of the cat mRNAs studied were intrinsically unstable, having half-lives between 4 and 14 min. The translatability of the cat mRNAs did not correlate with their half-life, and their decay was not blocked by cycloheximide. Therefore, the rapid degradation of the cat mRNA does not seem to depend on translational elongation and is not related in any obvious way to the rate of translational initiation. Furthermore, sequences in the 3'-trailer do not program the rapid decay of the cat mRNA. We discuss the implications of these data in the light of current models of mRNA degradation pathways.