The relative rates of degradation of two major tRNASer species in rooster liver were simultaneously assessed during induction by estradiol-17 beta benzoate of the synthesis of a serine-rich phosphoprotein, vitellogenin. The relative rate of degradation was determined by an in vivo pulse-chase labeling method, which included a 24-h labeling period with [5-3H]orotic acid prior to and a 6-day chase period with nonradioactive orotic acid after the administration of estrogen. tRNA Ser(AGU,C) and tRNASer (UCU,C,A) were extensively purified by chromatography on benzoylated DEAE-cellulose in the presence and absence of Mg2+ and their radioactivities determined. In three separate labeling experiments, the difference in radioactivity of pulse-labeled and chased tRNASer (AGU,C) vs. that of tRNASer(UCU,C,A) was approximately 2-fold, suggesting a slower rate of degradation of tRNASer(AGU,C) during vitellogenin induction. Calculation of the approximate half-lives of the two tRNASer species indicates that the half-life of tRNASer (AGU,C) was increased from 3.1 days to 6.2 days during vitellogenin induction, while that of tRNASer (UCU,C,A) was essentially unchanged (2.6 days). Regulation of tRNA degradation which is possibly connected with the frequency of its use in ribosomal protein synthesis may help to explain why, in many differentiated cells, the tRNA population is adapted to the amino acid composition of the synthesized proteins.