The abundance of the mRNA for human triosephosphate isomerase (TPI) is decreased to approximately 20% of normal by frameshift and nonsense mutations that cause translation to terminate at a nonsense codon within the first three-fourths of the reading frame. Results of previous studies inhibiting RNA synthesis with actinomycin D suggested that the decrease is not attributable to an increased rate of cytoplasmic mRNA decay. However, the step in TPI RNA metabolism that is altered was not defined, and the use of actinomycin D, in affecting all polymerase II-transcribed genes, could result in artifactual conclusions. In data presented here, the nonsense codon-mediated reduction in the level of TPI mRNA is shown to be characteristic of both nuclear and cytoplasmic fractions of the cell, indicating that the altered metabolic step is nucleus associated. Neither aberrancies in gene transcription nor aberrancies in RNA splicing appear to contribute to the reduction since there were no accompanying changes in the amount of nuclear run-on transcription, the level of any of the six introns in TPI pre-mRNA, or the size of processed mRNA in the nucleus. Deletion of all splice sites that reside downstream of a nonsense codon does not abrogate the reduction, indicating that the reduction takes place independently of the splicing of a downstream intron. Experiments that placed TPI gene expression under the control of the human c-fos promoter, which can be transiently activated by the addition of serum to serum-deprived cells, verified that there is no detectable effect of a nonsense codon on the turnover of cytoplasmic mRNA.