An attempt was made to define the importance of accumulated carcinogen dose and of induction-time on the development of pancreatic tumors after administration of N-nitrosobis(2-hydroxypropyl)amine (BHP) to Syrian golden hamsters. The development and progression of ductal hyperplasia, the number of tumor-bearing animals, the number of cancer-bearing animals, the number of tumors per tumor-bearing animal, and the grade of histological differentiation were found to be related to the accumulated dose of the carcinogen given, as was survival-time. The duration of the period between the carcinogen administration and examination was another factor of importance for the same changes except for the grade of histological differentiation of cancers and for survival-time. Once ductal hyperplasia and tumor development had started, there were no signs of regression - but rather progression - even after cessation of carcinogen administration. It is concluded that the development (progression) of induced pancreatic cancer in the Syrian golden hamster is influenced by the accumulated carcinogen (BHP) dose and the duration of the experiment. The results emphasize the need for a strict standardization of the experimental design as well as for the histological evaluation when animal models are used for studies of the effect of factors or procedures with a possible influence on the rate of tumor development.