Providing the most appropriate and effective anaesthetic regimen is an essen-tial part of good experimental design. Anaesthesia has profound effects on the physiological processes of animals, and this can have a marked effect on experi-mental data. These effects can arise as a direct result of the anaesthetic agents used, for example, hyperglycaemia caused by medetomidine. Other effects such as hypothermia may be secondary to the depression of various body systems. Some effects persist only during the period of Anaesthesia, other effects may continue for hours or days. Most experiments that use living animals aim to pro-duce a carefully defined abnormality, or to carry out a procedure such as catheter implantation. When such interventions have been completed, the goal is often to have no other significant effects on the animal’s physiology. Achieving these goals can be frustrated by the use of inappropriate anaesthetics and a failure to provide high standards of peri-operative care. Sometimes, the effects of a poor choice of anaesthetic agent can be dramatic, for example, ileus (gut stasis) after administration of chloral hydrate. More usually, the effects are less obvious, but even subtle changes can result in an increase in the variability of study data. This increased variability can require an increased number of animals to be used to demonstrate treatment effects (Festing et al., 2002). 
Aside from the requirements to carry out studies as efficiently as possible, it is generally accepted that when it is still necessary to use animals in experi-ments, research procedures should be refined to minimize pain and distress. A requirement to comply with the principles set out by Russell and Burch (1959), of Reduction, Refinement and Replacement, now forms part of the legislation controlling the use of animals in research in the UK and elsewhere.
Reviewing our anaesthesia and peri-operative care so that they are the most appropriate for a particular study will contribute to both reduction and refinement of animal use. 
Achieving this is not always straightforward, and it is important that attention should be given not only to the anaesthetic agents used, but also to the measures adopted to minimize the unwanted side-effects of anaesthesia and surgery. The  se of effective analgesia following surgical procedures is particularly important, yet recent reviews of current practices in rodents suggest that analgesic use is still relatively low (Richardson and Flecknell, 2006; Coulter et al., 2009). This is somewhat ironic, in that almost all of the analgesic and anaesthetic techniques currently used in humans were developed and assessed in laboratory animals, 
before being accepted for clinical use in humans. We therefore have a very wide range of techniques and anaesthetic and analgesic agents available for use in laboratory animals. Careful consideration of the options available can lead to improvements both in the quality of scientific data obtained and in the welfare of the animals involved.