We have studied the progression of breast tumors induced by N-methyl-N-nitrosourea in Wistar/Furst rats during serial passages through isogeneic animals, using characteristic markers previously defined. The tumors contained various types of structure, which evolve from intraductal nodes to duct-like structures and then to invasive strands or cells. Most primary tumors were not invasive but became so in subsequent transfers. The distribution of invasive and noninvasive tumors in the various lineages was not random, implying that the tumors differed from their inception in the propensity toward invasion. In different lineages invasive structures appeared repeatedly and independently but with different frequencies, suggesting that they were produced by a stochastic mechanism with different probability of occurrence. The possible nature of this mechanism is discussed. This experimental system is useful for further studies of tumor invasion; the results obtained are relevant for understanding and evaluating the evolution of human breast cancer.