Intracarotid infusion of LH-RH to female rabbits stimulated a significant increase in plasma LH concentration in the jugular vein. This response varied with the reproductive state of the animal, with a greater release occurring in oestrous (spontaneous or oestrogen-induced) and non-receptive does than in pseudopregnant or ovariectomized animals. If ovariectomized rabbits were pretreated with oestrogen, the pituitary response to LH-RH was restored. These findings suggest that there is little change in pituitary sensitivity to LH-RH infusion between oestrous and non-receptive rabbits, although pseudopregnancy (high physiological levels of progesterone) or ovariectomy inhibit its ability to respond to a releasing-hormone stimulus.