Mitogenic stimulation has been shown to increase both prostaglandin H (PGH) synthase-1 (PGHS-1) and PGH synthase-2 (PGHS-2) mRNA levels, although the time course and magnitude of induction are different for the two genes. To investigate the mechanism for mRNA induction, we conducted nuclear run-off assays of these two genes in 3T3 cells and correlated mitogen-induced changes in PGHS gene transcription with changes in PGHS mRNA and PGHS isozyme expression. We also examined the mechanism for glucocorticoid inhibition of PGHS mRNA expression and the effects of glucocorticoids on PGHS isozyme expression. Serum stimulation of quiescent 3T3 cells led to a sequential increase in PGHS-2 gene transcription, PGHS-2 mRNA, and PGHS-2 enzyme levels. PGHS-2 gene transcription increased over 25-fold within 30 min of serum addition resulting in an over 70-fold increase in PGHS-2 mRNA by 1 h, and maximal PGHS-2 enzyme expression by 2 h. Increased PGHS-2 isozyme expression thus appears to depend on transcriptional activation of the gene. Transcription of the PGHS-2 gene declined after 30 min, and PGHS-2 mRNA levels declined similarly after 1 h, leading to a return of PGHS-2 levels to near basal levels by 6 h. Glucocorticoids, which previously have been shown to inhibit mitogen-stimulated increases in PGHS-2 levels, were found to inhibit serum-stimulated increases in PGHS-2 gene transcription by 70%, resulting in a 70% reduction in peak serum-stimulated PGHS-2 mRNA levels also. Western blotting with PGHS-2 specific antisera demonstrated that while dexamethasone simply reduced PGHS-2 mRNA levels, it completely suppressed expression of PGHS-2 protein. The coincidental reduction in PGHS-2 transcription, PGHS-2 mRNA, and enzyme levels by dexamethasone, provides further support for the hypothesis that control of transcription is one primary control mechanism for regulating PGHS-2 expression. That complete suppression of PGHS-2 enzyme expression occurs following partial suppression of PGHS-2 mRNA, however, suggests that other mechanisms may also contribute to the glucocorticoid effect. A small, but reproducible, increase in transcription of the PGHS-1 gene occurred 3 h following serum stimulation, coincident with a three- to fourfold increase in PGHS-1 mRNA; PGHS-1 mRNA remained elevated for at least 3 h. Dexamethasone reduced, but did not completely inhibit, the serum-stimulated increases in PGHS-1. However, changes in PGHS-1 mRNA were not accompanied by detectable changes in PGHS-1 protein in the presence or absence of dexamethasone.