In these studies, prostaglandin H synthase activity was increased in stably Harvey-ras transfected C3H10T1/2 cells. The level of total prostaglandin H synthase protein was two-fold higher in stably Harvey-ras transfected C3H10T1/2 cells than in control cells with no difference in prostaglandin H synthase-2 level. Prostaglandin H synthase-1 mRNA level was two-fold higher in transfected than in control cells, while prostaglandin H synthase-2 was not significantly different. Thus, prostaglandin H synthase-1, but not prostaglandin H synthase-2, expression was increased in Harvey-ras transfected C3H10T1/2 cells.