Using a method for nitrosamine analysis that gives high recovery values and that is free from artifactual synthesis of nitrosamines, we have shown that human feces do not contain volatile nitrosamines (detection limit, 0.1 to 0.5 microgram/kg). We also showed that nitrosation reactions are not catalyzed by fecal organisms. Following a 2-day anaerobic incubation of feces with either a secondary amine (dimethylamine, dipropylamine, or morpholine) or nitrite, no nitrosamine was formed. When the amine and nitrite were added together, nitrosamine was formed, but at a level of 2 to 20% of that formed in autoclaved feces under the same conditions. Nitrosamines were stable following anaerobic incubation with feces for up to 4 days. These results suggest that fecal organisms inhibit the chemical formation of nitrosamines instead of catalyzing it. When morphine and nitrate were added together, nitrosomorpholine was formed. Morpholine nitrosates so rapidly that it intercepts nitrite formed by the action of nitrate reductase before the nitrite can be further reduced. However, very high concentrations of morphine and nitrate, which are far from the conditions in normal feces, were required to form measurable nitrosomorpholine. We may conclude that N-nitroso compounds are unlikely to be formed in any significant amounts in the human colon.