Social interactions in solitary animals, such as Heermann’s kangaroo rat (Dipodomys heermanni), have been little studied. In three experiments, the effect of social housing on levels of aggression and maintenance of estrous cycling was studied as part of a captive breeding program for an endangered form of this species. Expt 1 demonstrated that mixed-sex pairs of rats that were neighbors in social housing were no friendlier during staged encounters than unfamiliar pairs also housed socially. In expt 2, rats that had been isolated for several months were transferred to social housing and paired at weekly intervals with an opposite-sex neighbor. Initial encounters were highly aggressive and socialized partners retreated from previously isolated rats more often than vice versa, regardless of sex. By the third week, encounters resembled pairings of fully socialized rats, as in expt 1. Thus, social exposure may produce a general socializing effect rather than promote familiarity between specific individuals. In expt 3, the clear, perforated barriers that separated rats in social housing were replaced with solid, opaque barriers for 6 wk. Estrous cycle length immediately doubled and cycles of most females were disrupted for months. Taken together, the results reveal the influential role of social contact in this solitary species and highlight the need for more such work both for developing comprehensive theories of sociality and for guiding captive propagation efforts.