The distribution and metabolism of N-nitroso[14C]morpholine ([14C]NMOR) in Sprague-Dawley rats were studied by autoradiographic and in vitro techniques. Low-temperature whole-body autoradiography indicated that the non-metabolized compound is able to pass freely through the cellular membranes and distribute evenly in most tissues of the body. Experiments in vitro showed that, in addition to the liver, the nasal mucosa had a high capacity to degrade the [14C]NMOR and a localization of metabolites was also observed in this tissue in vivo. Microautoradiography of the nasal olfactory mucosa showed the highest labelling over the subepithelial glands in the lamina propria mucosae. The nasal mucosa is, apart from the liver, the most prevalent site of NMOR carcinogenesis in the rat and the results indicate that a local bioactivation occurs in this tissue. Further experiments showed that NMOR metabolism in the nasal mucosa and the liver was inhibited by metyrapone and by a carbon monoxide atmosphere, indicating that the metabolism is cytochrome P-450 dependent. In addition to the nasal mucosa and the liver, a localization of metabolites was demonstrated in the oesophageal mucosa in vivo, while in vitro this tissue showed a capacity to degrade the [14C]NMOR. A low incidence of tumours of the oesophagus has been reported in rats treated with NMOR. It is concluded that NMOR is metabolized in the nasal and oesophageal mucosa as well as in the liver, and that local bioactivation at these sites may give rise to tumours.