The present study aimed to characterize the adrenal response to ACTH. A model was developed that coupled the non-linear disposition of cortisol with a physiologically-based model for cortisol secretion by the adrenals. It was assumed that the response to ACTH resulted from two mechanisms: a stimulation of the cortisol secretion rate and control of the duration of the secretion. Seven dose levels of ACTH were tested in horses, a species similar to man as regards adrenal function. The main result was that the secretion rate of the adrenal gland can be modelised by a zero-order process that is maximal for a relatively low dose of ACTH (0.1 µg.kg^-1). Beyond this dose, the increasing adrenal gland response is only due to the prolongation of the time of its secretion. The consequences of these different features were explored by simulation to reproduce classical pathophysiological situations encountered in man. Our model was able to reproduce and simply explain many adrenal gland responses that are dimmed by the different non-linearities of the system.