Abstract. The mouse outbred stock Fzt:DU has been bred in the Research Institute for the Biology of Farm Animals Dummerstorf, Germany for about 30 years. This paper describes the history and the development of some traits in this stock over 128 generations. It has been used as base population for several lines long-term selected for fertility, growth, fitness and behaviour and has been bred with an average number of 200 breeding pairs per generation using a rotational mating scheme. A simulation study was employed to investigate the effect of genetic random drift on the allele frequencies. The change of the drift variance, the probabilities for allele losses and the development of the effective population size over generations are represented. The effective population size was relatively high, compared to other mouse experiments worldwide, however the genetic variability of the Fzt:DU population is substantially reduced due the high number of generations of isolated reproduction. After 120 generations, the variance effective population size is reduced to approximately 3 animals.