Formaldehyde (FA) is suspected of being associated with the development of leukemia. An inhalation experiment with FA was performed in rats to study whether FA can enter the blood and could thus cause systemic toxicity in remote tissues such as the bone marrow. Therefore, a sophisticated analytical method was developed to detect blood concentrations of FA during and after single 6-h exposure by inhalation.In order to differentiate between exogenous and endogenous FA the rats were exposed to stable isotope (13C) labeled FA by inhalation. During and after exposure of the rats to 13C-FA their blood was analyzed to determine the ratio between labeled and natural FA in blood and the total blood concentration of FA. With respect to sensitivity, with the applied method exogenous 13C-FA could have been detected in blood at a concentration approximately 1.5% of the endogenous FA blood concentration.Exogenous 13C-FA was not detectable in the blood of rats either during or up to 30min after the exposure. It was concluded that the inhalation of 13C-FA at 10ppm for 6h did not result in an increase of the total FA concentration in blood. © 2012 Elsevier Ltd.