The intermittent type of drug action which is desired in hypnotic treatment is fundamentally different from those types of drug treatment where a constant effect is required. Thus, in insomnia, drug action should be restricted to the duration of the night and residual effects should be absent during day-time. During daily administration there should be no accumulation of the drug. These factors taken together, mean that a rapid rate of elimination is of advantage for a hypnotic. In addition, the patient taking a hypnotic expects that sleep is readily obtained and therefore biopharmaceutical factors, especially those promoting a rapid rate of absorption, are crucial for hypnotic drug formulations.Many barbiturates have a relatively long elimination half life, except methohexitone, hexobarbitone and cyclobarbitone. The traditional classification of barbiturates into long-, intermediate-and short-acting compounds bears no relation to the rate of elimination in humans, and for this and other reasons, should be abandoned. Barbiturate salts are rapidly absorbed, in contrast to the free acids. Liver disease tends to decrease the elimination rate of these compounds, whereas renal insufficiency may give rise to accumulation of polar metabolites (e.g. hydroxyamylobarbitone). Methaqualone, too, has a long elimination half life, so that accumulation may occur during daily administration. Absorption of this compound appears to be quite rapid from the formulations investigated.Although the benzodiazepines nitrazepam and ./lurazepam are the most frequently used hypnotics today, information concerning their pharmacokinetics in humans is very limited. The elimination half life of nitrazepam is between 18 and 34 hours, whereas that of unchanged ./lurazepam has not been accurately determined. However, the N-desalkyl metabolite of ./lurazepam, which has pharmacological properties comparable to the parent drug, has an elimination half life of 2 to 4 days. Substantial accumulation of this metabolite occurs during daily administration of ./lurazepam. The active metabolite of chloral hydrate (trichloroethanol) is quite rapidly eliminated (t /f2 = 7 to IOh), whereas another important metabolite (trichloroacetic acid, t 1/2 = 4 to 5 days) gives rise to substantial accumulation during chronic administration. The potential protein binding displacement interaction of this compound with coumarin oral anticoagulants such as warfarin, should always be considered in clinical practice.