Confirming the results of R. T. Hill, it was found that the gonadotropic hormone content of the rabbit pituitary was rapidly depleted in the 24 hours following mating, and that the restitution of the gland content followed a characteristic curve during pseudo-pregnany. Repeated assays over a 2-yr. period of glands from estrous rabbits, and rabbits at several stages of pseudopregnancy demonstrated a variation in the level of this entire curve, each point of which was highest in the early spring and lowest in early winter.