Summary. Transection of the uterine horns near to their cervical ends (posterior uterine section) led to a prolongation of pseudopregnancy from 13\m=.\1\m=+-\0\m=.\16 (control) to 14\m=.\9\ m=+-\ 0\m=.\24 days (P<0\m=.\001). This was significantly greater than the prolongation following transection of the uterine horns one third of the way from the uterotubal junction to the uterine bifurcation (14\m=.\0 \ m=+-\ 0\m=.\16 days, P<0\m=.\01).When unilateral ovariectomy was accompanied by contralateral posterior uterine section, the prolongation of pseudopregnancy ( 14\ m=. \ 5\ m=+-\ 0\m=.\20 days, control 12\m=.\8\ m=+-\ 0\m=.\25 days, P< 0\m=.\001) was greater than when accompanied by ipsilateral posterior uterine section (13\m=.\7 \ m=+-\ 0\m=.\20days, P<0\m=.\02).These results indicate a quantitative and a unilateral component in the effects of uterine section, and support the hypothesis that prolongation of pseudopregnancy following posterior uterine section is brought about by the same mechanism as that following hysterectomy.