The nature of the input signals to the adrenal gland looked upon as a "black box" and the gland's steady-state characteristics with respect to its input channels are analyzed in this correspondence; new experimental data as well as published data are considered. It is shown that the organ has two independent inputs, ACTH concentration of the arterial blood and blood flow through the gland; and, further, that at steady-state the adrenocortical output is linearly related to the absolute Palue of the blood flow under constant ACTH concentration of the adrenal arterial blood and linearly related to the logarithm of ACTH concentration under constant blood flow. This concept replaces the earlier notion of a linear relationship between adrenocortical steady-state response and the logarithm of the product of adrenal blood flow and ACTH concentration.