Intraocular pressure is mainly controlled by the rate of aqueous humor production and the resistance to its outflow. Aqueous humor formation is the result of fast unidirectional secretion and slow contradirectional reabsorption. The most striking evidence of endogenous regulation of the ciliary epithelial secretion is provided by the observation of the circadian rhythm. Aqueous humor formation is mainly controlled by the interaction of inhibiting alpha(2)-adrenoreceptors and stimulating beta-adrenoceptors. The role of the central nervous system in the regulation of this process remains unclear and the precise mechanism of outflow regulation is not fully understood. The aqueous humor passes into the anterior chamber and leaves the eye by two routes: the direct outflow pathway through the trabecular meshwork or the indirect outflow pathway through the ciliary muscle. Further biochemical, biological and pharmacological investigations are necessary to determine the fundamental process of aqueous humor formation, outflow and regulation.