Effect of the intermittent administration of N-nitrosopyrrolidine (NPYR) on the tumor incidence in Sprague-Dawley rats.

N-Nitrosopyrrolidine (NPYR) was administered orally to 320 Sprague-Dawley rats. An additional group of 80 rats served as an untreated control. There were four experimental groups in which the effects of different periods of dosing and varied intervals without treatment were compared. The total dose for all the groups was constant and was 792 mg/kg body weight. The schedules of carcinogen application had a significant effect on the observed cancer risk. The different tumor indicences in the individual groups could not be attributed to differences in the survival pattern, but were related to the changes in the different treatment schedules.