A semiautomated three-compartment box (3CB) for studying partner preference behavior of rats is described. This apparatus automatically records the rat's time spent in each compartment, as well as the locomotor activity (i.e., the number of visits an animal pays to each compartment). Software was developed for calculating partner preference scores. Behavioral testing in the semiautomated 3CB, which is a modification of an earlier version, is less time consuming and less laborious. Three 3CBs can be observed simultaneously by two trained observers, and the behavioral interactions of three experimental animals with the stimulus animals can be observed and scored by hand. The use of the new apparatus was validated by studying adult partner preference behavior of neonatally ATD-treated male rats. The collected data fully corroborate previous results, obtained in the earlier version of the 3CB, again revealing the behavioral bisexual nature of these ATD males. A new finding was the much higher locomotor activity of the ATD males compared to controls.