The instillation of progesterone into the lumen of one uterine horn of unanaesthetized non-pregnant rabbits caused a more profound loss of intra-uterine pressure in the treated than in the placeboinjected control horn. The recovery of active pressure by the treated horn was significantly slower than by the control. The magnitude of the effect of progesterone, but not the rate of onset, was dose dependent. When progesterone was administered to one uterine horn of pregnant rabbits, asymmetrical delivery ensued, in which the control horns invariably expelled most of their foetuses before the treated horns commenced delivery. Autoradiographic study indicated that progesterone is very rapidly absorbed from the uterine lumen. The implication of these findings is discussed in relation to previous work.