Intravenous agents used in anaesthetic practice comprise three different groups of compounds: intravenous general anaesthetics, narcotic analgesics and skeletal muscle relaxants. All these drugs are characterised by a rapid onset of action and a short duration of action. These properties are the result of a number of common physicochemical and pharmacokinetic characteristics. The high lipid solubility of general anaesthetics and of narcotic analgesics explains their rapid onset of action when they are administered intravenously. The short duration of action corresponds to a rapid decline of the plasma concentration due to tissue distribution for most agents. The influence of elimination processes must also be considered, especially for long lasting operative procedures at the end of which a high cumulative dose of narcotic analgesics and muscle relaxants has been administered.Knowledge of the pharmacokinetics of the agents used may be of some help in the understanding of two serious anaesthetic accidents: (1) hypotension and cardiac.arrest during induction of anaesthesia which is due to an overdose of intravenous general anaesthetic; and (2) respiratory depression and respiratory arrest which can be attributed to a prolonged effect of narcotic analgesics and muscle relaxants. Thus, special attention must be focussed on the factors influencing the initial blood concentration for general anaesthetics and those influencing elimination processes for narcotic analgesics and muscle relaxants.For thiopentone, an excessive initial blood concentration results in hypotension. It may be related to an excessively rapid rate of injection, a reduced volume in which the drug initially distributes and/or a reduced protein-bound fraction. Distribution processes will mostly influence the duration of action of anaesthetics. Hepatic biotransformation of general anaesthetics will hardly influence their duration of action because of the relatively long elimination half-life of about 2 to 6 hours except for althesin, this drug being more rapidly metabolised with an elimination half-life of 0.5 hours.In contrast, elimination processes may influence the duration of action of narcotic analgesics and muscle relaxants. Knowledge about the pharmacokinetics of narcotic analgesics is incomplete and contradictory, especially for morphine. The elimination half-lives of morphine, pethidine (meperidine), pentazocine and fentanyl are similar (2.5 to 3.5 hours). Narcotics are eliminated by hepatic biotransformation. In liver parenchymal disease, prolongation of the elimination half-life has only been demonstrated for pethidine.In contrast to general anaesthetics and narcotic analgesics, muscle relaxants are highly hydrophilic compounds. Their volume of distribution is limited approximately to that of the extracellular fluids. Non-depolarising muscle relaxants are mainly excreted unchanged through the kidneys according to an ultrafiltration process. Biliary excretion represents an accessory pathway of excretion for all these compounds, except gallamine. Hepatic biotransformation is negligible. Their elimination half-lives are of about 2 to 3 hours. Higher values have, however, been reported for d-tubocurarine. Renal failure is the main cause of prolongation of action of non-depolarising relaxants. A less important cause of prolongation of the elimination half-life, for at least some relaxants, is the presence of hepatic parenchymal disease or cholestasis.