Analysis and knowledge of individual strain susceptibility of experimental animals to induction of carcinogenesis is important especially in regard to possibility of transfer of these facts to human pathology, first of all to chemopreventive projects. Our group (AHLERS et al. [1]) reported very low sensitivity of female Wistar:Han rats to induction of mammary carcinogenesis by 7,12-dimethylbenz(a)anthracene (DMBA) and by N-methyl-N-nitrosourea (NMU). The aim of this paper was to increase the sensitivity of females of this strain to mammary carcinogenesis induction by repeated administration of NMU in a dose 50 mg/kg of b.w. in critical periods: on 3-4 postnatal days, on 21 day (critical period for development of ductal parts of mammary gland) and between 50-55 days (maximal proliferation of whole gland). In comparison with 38% incidence of mammary tumors after the single dose and 65% incidence after 3 subsequent doses between 50-60 days, the combination of administration (only) on 21 day and between 50-55 postanatal days resulted in 88% incidence the sensitivity of animals reached the level of highly susceptible rat strains. The latency period was significantly increased in groups with NMU given on 3-4, 21 days and between 45-55 days respectively, on 21 day and between 45-55 days in comparison with control group (one dose of NMU). The tumor frequency per group and per animal in all groups with repeated NMU administration was significantly higher than that of control group. The volume of tumors was not influenced either by repeated carcinogen application or by time of its administration. These results expand the possibilities of analysis of carcinogen effects in individual periods of rat postnatal development.