Groups of male MRC Wistar rats were treated for 2 years either with morpholine (10 g/kg food) together with sodium nitrite (3 g/l drinking water) or with N-nitrosomorpholine (NM, 0.15g/l drinking water). In both cases, a group of rats was given sodium ascorbate (22.7 g/kg food) in addition to these treatments. When ascorbate was present, the liver tumors induced by morpholine and nitrite showed a 1.7-fold longer induction period, a slightly lower incidence, and an absence of metastases in the lungs, indicating that ascorbate had inhibited the in vivo formation of NM. Ascorbate did not affect liver tumor induction by the performed NM. The group treated with morpholine, nitrite, and ascorbate had a 54% incidence of forestomach tumors, including an 18% incidence of squamous cell carcinomas, possibly because ascorbate promoted NM action in this organ.