The present investigation examined, by study of daily vaginal smears, whether the interaction of virgin female rats with young affected the oestrous cycle. Virgins behaving non-maternally or maternally were continuously exposed to young for extended periods. (1) All non-maternal virgins continued to cycle while 87% of maternal virgins ceased to cycle when exposed to young, and entered a series of prolonged dioestrous phases, each lasting about 14 days. Likewise, all maternal parous non-lactating controls exposed to young entered prolonged dioestrus. (2) The dioestrous phase induced by young in virgins was characterized by the decidual response. (3) Young aged 4-9 days were as effective as 10-15-day-old young for inducing and maintaining the dioestrus. (4) Confinement of young in a wire-mesh basket showed that body contact with young was essential to establish dioestrus. (5) Body contact with young was sufficient to induce dioestrus in maternal virgins with excised nipples. (6) Once established, the dioestrus could be maintained for about 12 days after removal of young as early as Day 4 of dioestrus in virgin and parous non-lactating females, but removal of young on Day 6 from post-partum lactating females resulted in a rapid resumption of cyclicity. (7) Prolonged suckling during a series of dioestrous phases induced lactation in 61% of maternal virgins, as well as in all parous control females.