Ammonia labeled with cyclotron-produced 13N was injected intravenously in rats and the content of 13N in 14 major organs and tissues was determined at eight intervals ranging from 0.2 to 50 min after injection. The distribution of 13N at 12 s was employed to estimate the unidirectional tissue extraction for ammonia. The estimated fractional extraction for most of the tissues studied ranged from 70 to 100%. The 12-s 13N concentrations in a number of tissues (with lungs and brain the principal exceptions) were found to be quite similar to those reported for 42K+ and 86Rb+ in the rat, suggesting a similar mechanism to transcapillary extraction. Most of the injected dose was initially extracted by the musculature, lungs, and kidneys. The lungs and kidneys released the bulk of their extracted ammonia-derived nitrogen within 10 min of injection. The gut, heart, and spleen also recirculated extracted nitrogen, but on a much smaller scale than the lungs and kidneys. The recirculated label was accumulated mainly by muscle, liver, and skin. The results suggest that the lungs and kidneys are important sources of systemically recirculated ammonia metabolites in the rat.