The kinetics of induction and growth of acinar cell lesions has been investigated in rat pancreas after a single dose of the carcinogen azaserine. The time--response relationship was studied in male Wistar-related rats given a single i.p. injection of 30 mg L-azaserine/kg body weight at 18 days of age. Rats were killed between 4 and 78 weeks after treatment and ATPase-stained pancreas sections were quantitatively evaluated for the number and size of acidophilic, ATPase-positive and basophilic, ATPase-deficient foci. The number of acidophilic foci remained constant from 8 weeks onwards, while the number of basophilic foci slightly increased with time. The size of both acidophilic and basophilic foci increased throughout the experimental period. Due to two times higher number/cm3 and faster growth of the acidophilic foci, four times more acidophilic than basophilic focus tissue was present at the end of the experiment. Progression of acidophilic foci to adenomas and carcinomas was occasionally seen at later time points (greater than 34 weeks) in this rat strain. The dose--response relationship was studied in male and female Sprague--Dawley rats given a single i.p. injection of 0-45 mg azaserine/kg body weight at 19 days of age. Rats were autopsied at 17 weeks after treatment, and pancreas sections were quantitatively evaluated after ATPase histochemistry. The relationship between dose and number of foci was linear up to 30 mg/kg azaserine for both acidophilic and basophilic foci in males and females. For each individual dose, the number of foci induced was the same in males and females, and there were two to three times more acidophilic than basophilic foci. The percentage of pancreatic tissue occupied by focus tissue was 1.75 times higher in males, pointing to a higher growth-potential of acidophilic foci in males than in females. The first-order dose--response kinetics indicate that the conversion of a normal acinar cell into a focus-forming cell occurs by one specific azaserine-mediated rare event, occurring probably at the genetic level of the target cell.