The efficacy of various azaserine treatment durations was evaluated with respect to induction of atypical acinar cell nodules in Wistar rat pancreas and was related to animal age and rate of pancreatic DNA synthesis during growth. The sensitivity to nodule induction was maximal in postnatal rats when the rate of pancreatic DNA synthesis was high, whereas treatment of weanlings was less effective and treatment of mature rats was least effective. When weaned growing rats were given 1, 3, or 5 weekly injections of 30 mg azaserine/kg, the number of nodules induced was proportional to the number of injections. A single dose at this level did not induce detectable pancreatic necrosis or inflammation; therefore, DNA synthesis due to regeneration was probably not a major factor in the initiation of nodules. We concluded that multiple daily injections of [3H]thymidine during the first or second postnatal week provided DNA of sufficiently high specific activity for use in DNA repair and biochemical toxicity studies.