Restricting male rats to one-half normal food intake from day 25 until day 60 of life significantly delayed all aspects of normal growth that were measured. Body weights were reduced by 50% and pituitary growth hormone (GH) and plasma GH levels were severly reduced compared with corresponding values of ad libitum controls. Underfeeding also partially inhibited testicular and accessory organ growth. If underfeeding was combined with blinding, a more marked inhibition of testicular and accessory organ growth occurred. The effect of combining blinding and underfeeding was more dramatic than the effect of each treatment independently, indicating an accentuation of the response with the combination of treatments. This marked lag in development of the reproductive organs of blinded underfed rats was apparently mediated by the pineal gland, since blinded-pinealectomized underfed rats had reproductive organs that were considerably larger than those of blinded underfed rats.