HESC were treated with RA (10 μm), TPA (500 nm), RA + TPA or H2O2 (200 μm, as a positive control) for up to 2 h. GSH and GSSG concentrations were measured via HPLC and these values were used to calculate the cellular GSH/GSSG redox potential (Eh) through the Nernst equation (Jones, 2002). Results show that RA and RA + TPA, but not TPA alone, cause a significant oxidation (becoming more positive) of the GSH/GSSG Eh. Data shown are the result of at least 3 independently performed experiments for each treatment condition and are expressed as means (±s.e.m.). *Significant increase from initial (time zero) Eh values.