SUMMARY The pars distalis, pars intermedia and infundibular process make up, respectively, 61, 5 and 34% of the volume of the pituitary gland of normal mature female rhesus monkeys. The pars distalis is relatively smaller in immature animals, and larger during pregnancy. Section of the pituitary stalk is followed by a significant increase in the volume of the pars intermedia; the infundibular process always decreases in volume, while the pars distalis shrinks to a lesser and more variable extent. Histological changes in the pars intermedia suggest that cellular hypertrophy, infiltration of atypical cells, and cyst formation all contribute to the increase in volume, and that the changes are at any rate partly degenerative in nature.