Leghorn males, 66 to 96 days of age, were fed a diet marginally deficient in phosphorus (.14% nonphytate). Growth and feed intake were significantly reduced, but plasma calcium and phosphorus were unchanged. Bone ash was significantly reduced compared to pair-fed controls but not ad libitum-fed controls. Thyroid size increased and plasma growth hormone (GH) levels decreased with the deficiency, apart from any effect attributable to the lower food intake and weight gain. Plasma 3,5,3'triiodothyronine (T3) and thyroxine (T4) were not significantly changed, although there was a tendency for T4 to be lower in the deficient chickens. As both thyroid size and plasma GH are regulated partly by pituitary function, dietary phosphorus may alter endocrine function through changes in pituitary metabolism.