Rat pituitary acidophils consist of somatotropes (GH+/PRL-), lactotropes (GH-/PRL+), and lactosomatotropes (GH+/PRL+). Studies have indicated interconversion of these cell types in response to changing hormonal status. Representative tumor cell lines have been obtained for each acidophil cell type, and some display spontaneous interconversions. We examined whether the switch from GH3 cells (GH+/PRL+) to GH3LP and GC cells (both GH+/PRL-) involves repression of PRL gene expression at a transcriptional vs. posttranscriptional level. PRL mRNA is undetectable or barely detectable in GH3LP and GC cells. In contrast, nuclear extracts from these cells transcribe the PRL promoter in vitro, and their Pit-1 mRNA levels are comparable to those in GH3 cells. Nuclear run-on transcription assays demonstrated that the PRL gene is transcribed in GH3LP and GC cells at a rate of about 60% of that observed in GH3 cells. No evidence was obtained for a block to transcriptional elongation or for transcription in the antisense direction across the PRL gene. Northern blot analysis of nuclear RNA revealed partially degraded and undetectable PRL gene transcripts in GH3LP cells and GC cells, respectively. These findings indicate that PRL gene transcripts are specifically degraded in tumor cells which display a pure somatotrope phenotype and raise the possibility that the trans-differentiation of lactosomatotropes to somatotropes involves posttranscriptional regulation of PRL gene expression.