We have previously reported that a microcarrier-attached human hepatoma (Hep G2) cell line responds to hydrodynamic shear upon transfer to an agitated, clean, autoclaved spinner flask with a transient increase in cytochrome P450IA1 (CYPIA1) activity. Physiological changes induced by hydrodynamic stress could be problematic in the scaleup of microcarrier cultures. A better understanding of how stress alters cell physiology may assist in reactor scaleup. The induction of CYPIA1 activity was dependent on the agitation level of the cultures, and the level of CYPIA1 induction was comparable to that obtained with exposure to approximately 0.1 nM TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin). It has been well documented that hydrodynamic shear stress can cause alterations in the metabolism of phospholipid membrane-bound. arachidonic acid (AA) in adherent cells in a parallel plate system. The present study was carried out to determine if either AA or a metabolite of AA was involved in the induction of CYPIA1 activity in the microcarrier cultures of Hep G2 cells. Addition of exogenous AA followed by initiation of the stress resulted in an increase in the level of CYPIA1 activity. Pretreatment of the cultures with quinacrine, an inhibitor; of phospholipase A(2), reduced the stress-induced CYPIA1 activity. Furthermore, addition of propranolol, an inhibitor of phosphatidic acid phosphohydrolase, resulted in an increase in the response in addition to sustaining the induced enzyme activity. Pretreatment with the cyclooxygenase inhibitor, indomethacin, or the lipoxygenase inhibitor, caffeic acid, had no effect on the response, suggesting that the cyclooxygenase and lipoxygenase pathways were not involved in generating AA metabolites that alter CYPIA1 activity. The agent, nordihydroguaiaretic acid, blocks the monooxygenase pathway and blocks CYPIA1 activity increases. These observations suggest a possible mechanism where the stress on the cells induces phospholipase D, resulting in the formation of phosphatidic acid which then activates phospholipase A(2), resulting in the release of AA. Further, these results are consistent with a mechanism in which the metabolism of AA, most likely through the monooxygenase pathway, results in a metabolite that by a Stet unknown mechanism induced CYPIA1.