Rats were given 3H-labelled 4-ipomeanol intravenously and whole-body autoradiography with freeze-dried sections, or with sections extracted with trichloroacetic acid, water and organic solvents, was performed to examine the disposition of unbound and bound radioactivity in various tissues. Microautoradiography with glutaraldehyde-fixed, resin-embedded material was used to investigate the cellular distribution of bound metabolites. Based on the data obtained from these experiments in vitro incubations with tissue-slices were carried out to examine the capacity by various tissues to form tissue-bound 3H from the 3H-labelled 4-ipomeanol and autoradiography of isolated organs after incubation with 3H-labelled 4-ipomeanol was performed to study the localization of radioactivity under in vitro conditions. The results showed a high formation of tissue-bound 3H in the lung in vitro and a localization of bound metabolites in several structures of the lung in vivo. In vitro formation of tissue-bound 3H was also found in the nasal olfactory and respiratory mucosa, the hard palate, the trachea, the liver and the kidney and this was also correlated with a localization of bound 3H in these tissues in vivo. Incubations of the lung, the nasal olfactory mucosa, the hard palate and the liver in CO- or N 2-atmospheres or in the presence of the cytochrome P-450-inhibitor metyrapone showed decreased formation of tissue-bound 3H from the 3H-labelled 4-ipomeanol, indicating a role of cytochrome P-450 in the metabolism of 4-ipomeanol in the various tissues. The correlation between the in vitro capacity of various tissues to metabolize the 4-ipomeanol and the in vivo accumulation of tissue-bound metabolites in the same tissues indicate that a local bioactivation of the 4-ipomeanol takes place in these tissues in vivo. Our results indicate that the list of tissues with a capacity to metabolize 4-ipomeanol in vivo is as follows: Bowman's glands in the olfactory lamina propria mucosa, mucous cells in the nasal respiratory mucosa, the lateral nasal gland (Steno's gland), serous glands in the septum and the lateral wall of the nasal cavity, the mucosa in the nasopharyngeal duct and the pharynx, the ciliated and the mucous cells in the trachea and the bronchi, the Clara cells and the ciliated cells in the bronchioli, some cells in the walls of the alveoli, the middle and superficial cell layers of the stratified squamous epithelium in the hard palate, the liver with the highest metabolism in the central parts of the liver lobuli, and the kidney with active cells being present in the cortex. The bioactivation of 4-ipomeanol in the lung, the liver and the kidney has been shown to correlate with noxious effects. The results of the present indicate additional potential target tissues for the 4-ipomeanol.