The arotinoid Ro 15-0778 is a potent inhibitor of sebum secretion in rodents but not in humans, while isotretinoin (13-cis-retinoic acid, Accutane®) inhibits sebum secretion in rodents and humans. The concentrations of Ro 15-0778 and isotretinoin were determined in plasma and target tissues of castrated, testosterone-stimulated hamsters after oral and topical dosing and in castrated, testosterone-stimulated rats after oral dosing. Comparison of this concentration data to that obtained in the plasma and sebum of humans after dosing with Ro 15-00778 and with isotretinoin did not explain why Ro 15-0778 was a potent inhibitor of sebum production in rodents but not in humans