Determination of the concentration of drugs and metabolites in biological fluids or matrices other than blood or urine (most commonly used in laboratory testing) may be of interest in certain areas of drug concentration monitoring. Saliva is the only fluid which can be used successfully as a substitute for blood in therapeutic drug monitoring, while an individual's past history of medication, compliance and drug abuse, can be obtained from drug analysis of the hair or nails. Drug concentrations in the bile and faeces can account for excretion of drugs and metabolites other than by the renal route. Furthermore, it is important that certain matrices (tears, nails, cerebrospinal fluid, bronchial secretions, peritoneal fluid and interstitial fluid) are analysed, as these may reveal the presence of a drug at the site of action; others (fetal blood, amniotic fluid and breast milk) are useful for determining fetal and perinatal exposure to drugs. Finally, drug monitoring in fluids such as cervical mucus and seminal fluid can be associated with morpho-physiological modifications and genotoxic effects. Drug concentration measurement in nonconventional matrices and fluids, although sometimes expensive and difficult to carry out, should therefore be considered for inclusion in studies of the pharmacokinetics and pharmacodynamics of new drugs.