The regulation of urokinase plasminogen activator (uPA) expression was investigated in 2 highly metastatic rat mammary adenocarcinoma cell lines, BCI and MAT 13762. BCI cells were observed to synthesize, on average, 10 times less uPA enzyme and mRNA than MAT 13762 cells; however this difference was not accounted for by differences in uPA gene copy number/ structure or in the rate of uPA gene transcription in the cell lines studied. Moreover, Northern blot analysis of invasive sub‐populations derived in vitro from the BCI cell line revealed levels of uPA expression similar to those of the parent, but a 3‐fold elevation in expression of the metalloprotease gene, transin. Further investigation showed that treatment of BCI cells with either of the protein synthesis inhibitors, cycloheximide or anisomycin, increased the level of both nuclear and cytoplasmic uPA RNA 6‐ to 18‐fold in 4 hr, whilst inducing a maximum 2.6‐fold increase in the rate of uPA gene transcription. This increase in uPA gene expression may therefore reflect, in part, an increase in the stability and/or processing of nuclear uPA transcripts. These results suggest that the degree of uPA gene expression does not correlate directly with BCI tumor‐cell invasion in vitro, and that the uPA gene is down‐regulated, at least in part, post‐transcriptionafly in the nucleus of BCI mammary tumbr cells.