Plasma somatomedin (SM) activity and growth hormone (GH) concentration were measured in ten growth-retarded, well-nourished pediatric renal allograft recipients who received daily prednisone therapy. The SM activity ranged from 0.21 to 1.22 micron/mL, and it was subnormal in three patients. A significant correlation was found between SM activity and creatinine clearance. Serial SM levels were determined during a 24-hour period in five patients; SM activity decreased at six and 12 hours and returned to normal values by 24 hours. The 24-hour plasma GH concentrations ranged from 1.5 to 7.6 mg/mL. Peak GH concentrations ranged from 2.1 to 14.2 ng/mL after insulin-induced hypoglycemia and from 1.8 to 24.6 ng/mL after oral glucose loading. Sleeping GH peaks were absent in two patients. These results suggest that growth failure in renal allograft recipients who receive daily prednisone may result from (1) partial GH deficiency, (2) reduced SM levels owing to diminished allograft function, and (3) daily transient decrease in plasma SM levels after prednisone administration.