Studies by optical microscopy showed that discharge of neurosecretion brought about by the stimulus of milking took place in the zona centralis and also in the transitional and main sectors of the zona terminalis. The zona marginalis, zona metacentralis and the peripheral sector of the zona terminalis were not involved. In pituitary glands fixed at various time intervals after milking, the increase in secretion content and the disappearance of perivascular accumulations of secretion were evidently a result of renewed production and storage of neurosecretion in the hypophysis. The renewed homogeneous distribution of secretion in the zona terminalis had not been completed one hour after milking. The increase in size and number of pituicytes and their frequent appearance in adventitious tissue of vessels and around sinusoid capillaries after the liberation of secretion may be related to degradation processes of neurosecretory material.