This review deals with the concept of sleep mechanism based on our uridine receptor theory. It is well established that uridine is one of the sleep-promoting substances, we have, therefore, synthesized new types of hypnotic compounds from oxopyrimidine nucleosides. Their mechanism of action in CNS depressant effects is elucidated based on the receptor theory. In this study, structure-activity relationship for CNS depressant properties, sleep-promoting effects, interaction with certain CNS receptors, and receptor binding assay of uridine derivatives as oxopyrimidine nucleoside were investigated. In the studies of structure-activity relationship of N3-substituted uridine, we found for the first time that both N3-benzyluridine and N3-phenacyluridine synthesized exhibited potent hypnotic activity (loss of righting reflex) by intracerebroventicular injection in mice. Moreover, certain derivatives of these compounds possessed synergistic effects with barbiturate and benzodiazepine, and decreased in spontaneous activity, motor incoordination, and antianxiety effects in mice. Especially, N3-phenacyluridine markedly enhanced pentobarbital- and diazepam-induced sleep by 6- and 70-fold, respectively. However, N3-benzyluracil and N3-phenacyluracil that have no ribose moiety did not possess any hypnotic activity, indicating specific effects of nucleoside derivatives. Effects of N3-benzyluridine on natural sleep in rats were thus examined. N3-Benzyluridine also possessed the sleep promoting effect assessed by electrocorticogram at the dose of 10 pmol. For elucidating the mechanism of action of N3-phenacyluridine, the interactions of this compound with benzodiazepine, GABA, 5-HT, or adenosine receptors were also investigated. Although the pharmacological activity of N3-phenacyluridine was high, the affinities to benzodiazepine, GABA, 5-HT, and adenosine receptors were quite low. [3H]N3-Phenacyluridine concentration-dependently bound to synaptic membrane prepared from the bovine brain. The Scatchard analysis revealed a single component of the binding site. This binding site is proposed here as a novel receptor called "uridine receptor" for hypnotic activity of the uridine derivatives. The rank order of the distribution of these specific binding sites was found to be striatum > thalamus > cerebral cortex > cerebellum > mid brain > medulla oblongata in the rat brain. In the metabolic study of N3-phenacyluridine, we found that this compound was exclusively metabolized to N3-(S)-(+)-alpha-hydroxy-beta-phenethyluridine, but not the (R)- form, in mice. N3-(S)-(+)-alpha-Hydroxy-beta-phenylethyluridine possessed not only strong hypnotic activity but also a high affinity to the uridine receptor of synaptic membranes, while the (R)-isomer was low in both activities. Racemic mixture was shown to be intermediate for pharmacological effects of the compounds. These studies which used (R)- or (S)-isomer indicate that uridine binding site or uridine receptor, exists in the CNS and plays some role in sleep regulation in mammals as one of the triggering steps in inducing hypnotic activity. It is suggested that uridine is released from steps of nucleic acid-nucleic protein biosynthesis (catabolism), and reaches the binding sites in the areas of the brain which regulate natural sleep. The uridine dissociated from the receptor is then utilized for the synthesis of nucleic acid (anabolism). We propose here that the induction of sleep may be mediated by uridine through uridine receptor in the CNS, although the structure of uridine receptor is not yet elucidated.