The aim of the study was to investigate the concentrations of Ro 10–1670 (acitretin) and its isomeric metabolite Ro 13–7652 (cis-acitretin) after multiple oral dosing of acitretin. We used a highly sensitive HPLC method for simultaneous determination of the 2 retinoids with a quantification limit of 2 ng/ml in plasma and 10 ng/g in total skin (epidermis and dermis). In hairless rats receiving orally 8 mg/kg acitretin once daily during 8 days, blood and skin samples were taken at different time points between 5 and 96 h after the last dose. After 96 h, appreciable concentrations of Ro 10–1670, but not Ro 13–7652 could be measured in the skin, whereas both isomers were below the quantification limit in plasma. In psoriatic patients treated with a once daily dose of 30 mg acitretin, blood samples and biopsies were taken after 1 month of treatment (i.e. under steady state conditions). 24 h after the last drug intake, skin concentrations of acitretin were approximately 10 times higher than those observed in plasma. Ro 10–1670 concentrations in the skin were approximately 3–5 times higher than for Ro 13–7652 and concentrations of both isomers were higher in lesional compared to un