Stereotaxic implantation of 200μg cortisone pellet in agar were placed in the median eminence region in male albino rats of Holtzman strain. A group of cortisone implanted animals were exposed to cold stress (8±1°C)and the other group was kept in the animal house (25±2·C). After 3 and 5 days of implantation, the animals were autopsied and the changes in histology of the pars distalis and pars intermedia were studied. Hypertrophy of basophils was observed in the implanted group and the additional stress of cold resulted in gradual degranulation of ACTH cells by the third day and complete abolition of ACTH cells with infarction by the end of fifth day. A significant increase and degranulation of cells in the parts intermedia could be observed in the cortisone implanted, cold stressed group as compared to those kept in the animal house. These groups are compared with the pituitaries of sham operated groups.