Age-related decline of the thymus in mice was reversed by repeated inj
ections of bovine growth hormone. After hormone administration that wa
s begun at 5, 8, or 11.5 months of age, the cortical volume fraction (
Vc), cortical/medullary ratio (C/M) and thymocyte numbers found were i
n the range obtained with younger mice. Growth hormone injection reduc
ed the slope of the curves for Vc and C/M as a function of age, indica
ting a slowing down of the involution process.