Aggressive (TA) and nonaggressive (TNA) lines of mice were established by selective breeding for isolation-induced intermale aggression. This paper summarizes and updates studies performed on the TA and TNA lines. The genetic analysis revealed that in these lines the genes for aggression are located on the autosomes and demonstrate a Mendelian segregation. The genes are expressed only in the presence of androgens which are normally present only in males. Behavioral and biological responses associated with high and low levels of aggression in TA and TNA mice are reviewed. Line differences have been found in olfactory communication and marking behavior, in maternal and predatory aggression in females, in locomotor activity, and in learning abilities. Also, correlated neurochemical and endocrinological responses to the selection have been detected. Maternal factors during the preweaning period do not significantly affect the development of aggression in TA and TNA males, while early postweaning exposure to aggression or sex enhanced later aggressive and sexual activity. Early experience and genetic disposition for aggression are correlated, with TA males showing the greatest increase in the behaviors studied.