On the basis of the effects caused by 5-6-dihydroxytryptamine (5-6-DHT) on the serotonin nerve endings (chronic degeneration with depletion of the mediator), the eventual existence of serotoninergic fiber regulating pupillary diameter (PD) and intraocular pressure (IOP) was studied from a functional point of view. For the miotic effect found after administration of 5-6-DHT in rabbit, not accompanied by any change in the IOP, the hypothesis of a coparticipation of the serotoninergic fibers in the regulation of the PD is proposed, thus sustaining the concept that both PD and IOP respond to mechanisms of autonomous control. A phenomenon of receptorial supersensitivity to serotonin after the lesion of the nerve endings, dependent on 5-6-DHT, was also encountered.