It has been shown that biological rhythms influence the pharmacology and effects of anaesthetic agents such as local anaesthetics, hypnotics and muscle relaxants. This review discusses the latest findings and their consequences for anaesthesiological practice. RECENTS FINDINGS: Opioids and new local anaesthetics exhibit circadian changes when they are injected into spinal or epidural spaces for labour pain analgesia. Other studies have demonstrated that propofol and ketamine have maximal duration of action when they are injected during a period of rest in animals (at night in humans). It has been also shown that propofol can perturb the central circadian pacemaker and so cause a phase-shifted advance in effect on activity in rats.Although studies are lacking for most newer anaesthetic agents used in humans, recent findings emphasize once again that chronobiology should be considered in studies of anaesthetic drugs. Circadian rhythms should be considered in pharmacokinetic and pharmacodynamic analyses so that proper research protocols can be designed. The implications of chronobiology for the practice of clinical anaesthesia are probably of lesser importance because of the use of patient-controlled devices for pain management, monitoring of muscle paralysis and depth of anaesthesia monitors.