Physiological and morphological changes produced by corticotropin-releasing factor (CRF) stimulation in vivo were studied in pituitaries immunocytochemically stained for ACTH. After 48-h ip minipump infusions of 10 or 50 ng/min CRF, serum ACTH levels were increased significantly over values in control groups that included both intact rats and rats exposed to sham abdominal surgery. Correlative morphological changes included a striking increase in corticotrope cell area. This was coupled with an apparent decrease in the percentage of stained cells, probably due to degranulation. The cellular responses were similar to those after adrenalectomy described previously by us and others. Therefore, in a parallel study, additional groups of rats were adrenalectomized and studied 24 and 48 h after the surgery. Even greater changes in serum ACTH, corticotrope cell area, and percentages were observed in the adrenalectomized rats. The difference between the CRF-infused and adrenalectomized groups was probably due to the lack of corticosterone feedback in the latter group. Among the control groups, there were no differences between intact rats and rats exposed to sham abdominal surgery. Rats subjected to sham adrenalectomy, however, showed corticotrope responses similar to those of CRF-infused rats, except that the cells were more densely stained. The present studies thus show dramatic changes in ACTH cell area, extent of staining, and percentages after in vivo CRF stimulation. In all of the experimental groups, an excellent correlation existed between serum ACTH levels and the degree of the morphological changes in the corticotropes.