We determined the effects of phenol and 2,6-dimethoxyphenol (syringol) on N-nitrosomorpholine (NMOR) formation in rats given morpholine and nitrite by gavage. At 30 min post-gavage the recovery (from the stomach, duodenum and blood) of 564 micrograms NMOR was six times higher when administered to rats by gavage with 2 g of semipurified diet (SPD) than when given without food. Rats were gavaged with 12 mg each of morpholine, one of the modifiers and nitrite and examined 30 min later. Syringol decreased the amount of NMOR in both the stomach and blood by 89%, while phenol had no effect. We compared these results with those obtained with ascorbic acid and thiocyanate. The effect of ascorbic acid was similar to that of syringol. However, thiocyanate increased the amount of NMOR in the stomach and blood 2.7- and 4-fold, respectively. When 2 g of SPD was administered to rats by gavage, together with the precursors, syringol and ascorbic acid blocked NMOR formation in the stomach by 58 and 45%, respectively, and thiocyanate enhanced the yield 1.5-fold. The effect of phenol was not significant for the stomach and blood and that of the other modifiers was not significant for blood. Administration of the reactants together with food decreased the NMOR level in blood 155-fold relative to controls (no food), suggesting that food decreased the absorption rate over a 30-min period. These results demonstrate the modifying effect of phenol and syringol on NMOR formation in vivo to be similar to that observed in a previous in vitro study, and show that the effect of food on NMOR levels in blood was more important than that of the modifiers.