Two main classes of oral hypoglycaemic drugs, the sulphonylureas and the biguanides, are currently used in the therapy of type II, non-insulin-dependent diabetes mellitus (NIDDM). The basic pharmacokinetic properties of these agents are discussed with a view to efficient and safe treatment. Both first- and second-generation sulphonylureas are rapidly absorbed from the gastrointestinal tract. In the plasma compartment, these drugs are strongly bound to serum proteins. All sulphonylureas are metabolised in the liver, and the metabolites and the parent drugs are eliminated mainly in the urine, but also (second-generation derivatives) in the faces. Rapid- and short-acting sulphonylureas may improve early insulin release and promote better postprandial glucose control. Long-acting derivatives may ensure better control of overnight glycaemia. The elderly are at risk of developing severe sulphonylurea-induced hypoglycaemia, and in this population the agent chosen should have a short or intermediate duration of action and no active metabolites. Caution is needed when prescribing any sulphonylurea in patients receiving drugs known to affect sulphonylurea action, and in those with impaired liver and/or kidney function. The bioavailability of the biguanides ranges from 40 to 60%. Binding to plasma proteins is absent or very low. Metformin and buformin are not metabolised and are excreted in the urine; phenformin undergoes hepatic hydroxylation and is excreted in both urine and faeces. Metformin is the only agent of this class currently recommended for clinical use. The main indications of metformin treatment are NIDDM associated with obesity and/or hyperlipidaemia, and in combination with sulphonylurea both as primary treatment and when secondary failure occurs with sulphonylurea alone. Lactic acidosis may develop in patients receiving therapy with biguanides, especially in the presence of a preexisting contraindication to their use.