Abstract: We developed a procedure to measure mRNA decay rates in the yeast Saccharomyces cerevisiae and applied it to the determination of half-lives for 20 mRNAs encoded by well-characterized genes. The procedure utilizes Northern (RNA) or dot blotting to quantitate the levels of individual mRNAs after thermal inactivation of RNA polymerase II in an rpb1-1 temperature-sensitive mutant. We compared the results of this procedure with results obtained by two other procedures (approach to steady-state labeling and inhibition of transcription with Thiolutin) and also evaluated whether heat shock alter mRNA decay rates. We found that there are no significant differences in the mRNA decay rates measured in heat-shocked and non-heat-shocked cells and that, for most mRNAs, different procedures yield comparable relative decay rates. Of the 20 mRNAs studied, 11, including those encoded by HIS3, STE2, STE3, and MAT alpha 1, were unstable (t1/2 less than 7 min) and 4, including those encoded by ACT1 and PGK1, were stable (t1/2 greater than 25 min). We have begun to assess the basis and significance of such differences in the decay rates of these two classes of mRNA. Our results indicate that (i) stable and unstable mRNAs do not differ significantly in their poly(A) metabolism; (ii) deadenylation does not destabilize stable mRNAs; (iii) there is no correlation between mRNA decay rate and mRNA size; (iv) the degradation of both stable and unstable mRNAs depends on concomitant translational elongation; and (v) the percentage of rare codons present in most unstable mRNAs is significantly higher than in stable mRNAs.