On day 9 of dioestrus the uteri of pseudopregnant rats bearing deciduomata were either slit lengthwise to curette out the decidual tissue or were removed in toto to determine if the prolonging effect of decidualization on pseudopregnancy could be eliminated. Both operations significantly reduced the length of the pseudopregnancy dioestrus (combined mean of 19·3 ± 0·56 days v. 22·2 ± 0·77 days for rats with intact deciduomata; P < 0·01). Conversely, the control experiment of slitting the non-decidualized uterus on day 9 significantly prolonged pseudopregnancy in comparison with control groups (19·2 ± 0·92 days v. 13·5 ± 0·37 days for pseudopregnant controls which were not subjected to operation, or had a laparotomy or uterine traumatization performed on day 9; P < 0·001). The effect of slitting the uterus on other days of pseudopregnancy was investigated. The mechanism through which slitting prolongs pseudopregnancy is unknown.