The expected annual selection response is a function of selection intensity (i), generation interval (t) and precision (p) of breeding value estimation. These three parameters may, under certain conditions, be reduced to functions of one variable, y, the mean age of breeding animals at culling, which is a simple function of the annual replacement rate. When the number of breeding males and females are different, i and t become functions of two variables ym and yf, the mean ages of males and females at culling. The values of y (or of ym and yt) giving the maximum selection response are established for different domestic species: for individual selection, where the ratio i/t is to be maximized, for selection on a repeatable criterion connected to female reproduction (prolificacy, milk or egg production), where the maximum of ip/t is to be determined, and for male progeny-testing. The results show that, the more prolific a species is, the more rapid the renewal of breeding animals must be and that the renewal should be more rapid for males than for females, particularly in species of low prolificacy. When selecting on female performances which are repeated in time, the optimum renewal rate of females is lowered relative to individual selection on a criterion measured once. Numerical values have been calculated for different species, with given reproduction parameters for each one.