Intravenous administration of clonidine (1--100 micrograms/kg) produces a dose-dependent mydriasis in cats by inhibition of parasympathetic tone to the iris. The magnitude of CNS-induced pupillary dilation was similar in both normal anaesthetized cats and in anaesthetized preparations pretreated with reserpine (5 mg/kg i.p.) and alpha-methyl-p-tyrosine (2 x 300 mg/kg i.p.). Pretreatment reduced the concentrations of noradrenaline, dopamine and serotonin to less than 3% of that control levels in most parts of the CNS in which these amines were measured. Clonidine produced bradycardia in control animals but not in pretreated cats. In amine depleted animals in which only one eye was innervated by the ciliary nerves (parasympathetic), clonidine produced mydriasis only on the innervated side. These experiments confirm our previous observations that clonidine produces mydriasis in the cat by means of inhibition of parasympathetic tone to the iris. It is concluded that if clonidine produces this effect by stimulating noradrenergic, dopaminergic or serotonergic receptors, then clonidine exerts its centrally-induced mydriatic effect by acting on post-synaptic mechanisms.