Critical time period for the vagino-cervical stimulus to induce pseudopregnancy in the dioestrous rat.

Groups of rats experienced mechanical stimulation of the vagino-cervical region in induce pseudopregnancy at various intervals from 0900 h on dioestrus -- 1 (D -- 1) to 1800 h on D -- 2. Twenty-four of 26 rats which were stimulated on D -- 1 experienced immediate pseudopregnancy whereas when the stimulus was given on D -- 2, 39 of 45 rats continued to cycle and the latest time when such mechanical stimulation would reliably induce immediate pseudopregnancy was 2400 h on D -- 1. Abolition of the dark phase between 1800 h D -- 1 and 0600 h D -- 2 did no alter this critical time period (2400 h on D -- 1). Stimulation as late as 1200 h on D -- 2 caused alterations in the normal pattern of 4-day cyclicity in 8 of 11 rats.