['Investigation into the characteristics of anesthetic interactions may provide clues to anesthesia mechanisms. Dexmedetomidine, an α(2)-adrenergic receptor agonist, has become a popular sedative in intensive care, and hydroxyzine, a histamine receptor antagonist, is well known as a tranquilizing premedication for anesthesia. However, no experimental or pharmacological evaluation has been reported concerning their combination with propofol. Thus, we studied their combined effect with a hypnotic dose of propofol in ddY mice.', 'Male adult mice were intravenously administered either dexmedetomidine (30\xa0μg/kg) or hydroxyzine (5\xa0mg/kg) with propofol (3.75-10\xa0mg/kg) to induce hypnosis, defined as a loss of the righting reflex (LRR). Other mice were intravenously administered propofol, dexmedetomidine (300\xa0μg/kg), or hydroxyzine (50\xa0mg/kg) alone, and subsequent behavioral changes were observed. The 50% effective dose (ED(50)) for LRR was calculated, and the duration of LRR was determined.', 'The hypnotic dose of propofol was 9.95\xa0±\xa01.04\xa0mg/kg (ED(50)\xa0±\xa0SEM) without combination. Dexmedetomidine and hydroxyzine reduced the ED(50) of propofol to 5.32\xa0±\xa00.57 and 5.63\xa0±\xa00.57\xa0mg/kg, respectively. Coadministration of dexmedetomidine significantly extended LRR duration compared with propofol alone, whereas hydroxyzine significantly shortened LRR duration. A maximal dose of dexmedetomidine or hydroxyzine alone did not induce hypnosis.'] Do dexmedetomidine and hydroxyzine synergistically potentiate the hypnotic activity of propofol in mice?