chronobiology; ketamine/xylazine; cardiovascular system; autonomic nervous system; acid-base balance; ions; rat model

The creation of experimental, in vivo, chronobiological animal models may help to reveal some of the relationships between circadian rhythmsand biological function, which is sometimes very difficult to study in humans.However, in vivo experiments require the use of appropriate anaesthesia, which should be selected according to their particular effect on the organism. Using an animal model, the aim of the present study was to evaluate the chronobiological effects of ketamine/xylazine anaesthesia in spontaneously breathing rats. Study was performed in female Wistar rats (10mg/kg/15mg/kg, i.m)after adaptation to a LD cycle of 12 hours light:12 hours dark. Heart rate, rectal temperature, electrocardiographic parameters, autonomic nervous system activity, acid-base balance and Na+, K+, Ca2+ and Cl- plasma concentrations in arterial blood were evaluated in their dependence on the active (dark) and non-active (light) parts of the LD cycle.