Prothymosin alpha (PTA) mRNA and histone H4 (H4) mRNA levels were studied in various experimental conditions that affected GH1 pituitary tumor cell proliferation. Cell proliferation and progression through the cell cycle was assessed by counting cells, 3H-thymidine incorporation and flow cytometry. PTA mRNA levels were decreased in a time-dependent fashion following serum deprivation; when the cells were induced to grow by serum refeeding, PTA mRNA expression was greatly stimulated. Interestingly, after caprylic acid treatment (2.5 mM for 24 h) that arrested cells in the G0/G1 phase of the cell cycle, PTA mRNA and H4 mRNA levels were almost undetectable; conversely, following caprylic acid withdrawal, PTA mRNA and H4 mRNA expression were greatly stimulated. Furthermore, cells cultured in T3-deprived serum, which was found to decrease GH1 cell proliferation, had low levels of PTA and H4 mRNAs. This effect was reversed by the addition of nanomolar concentrations of T3 to the culture. On the other hand, IGF-1 addition to the culture did not substantially modify PTA mRNA levels. The present data clearly indicate that PTA mRNA expression is tied to the proliferating activity of GH1 cells and, thus, could be used as a marker of the action that various agents have on GH1 cell proliferation.