AbstractExperimental conditions simulating the induction of clinical pituitary gigantism and acromegaly were established by prolonged administration of growth hormone in high dosage to adult male rats starting at two different ages: 6 months (growth still active) and 14½ months (growth virtually arrested). Treatment continued for 14½ months, controls receiving saline injections. Each group numbered eight at onset. Standardized x‐rays of skull were made in ventro‐dorsal and lateral planes, at onset, mid‐period, and end of the study. Representative dimensions of cranial and facial segments were measured, including lengths, widths, palate dimensions, gnathic and interzygomatic angles, and incisor curvature. Some related indices were calculated. Means and standard errors were computed, usually on five to eight rats (oldest controls: three only). The response pattern of overall skull length was most illustrative. Younger adult controls grew actively until 14 months of age (5%) while injected rats grew still faster (8%); thereafter, controls grew negligibly (1%) and injected rats only slightly (2%). Older controls showed negligible skull elongation from 14½ to 29 months of age, and growth hormone stimulated no gain. In the younger group, skull length gains were almost entirely in the facial region; cranium gained no length and widened only slightly. Cranial index increased slightly with the hormone. Facial (bizygomatic) width increased in both injected groups—proportionately in younger rats (to gigantism) and disproportionately in older rats. Palatal and dental growth followed facial patterns in both groups. Cranial vault bones thickened and, in older rats, developed surface irregularities, giving them a more massive, acromegaloid