A rapid, simple and sensitive ion-pair HPLC method has been developed for quantification of metformin in plasma. The assay enables the measurement of metformin for therapeutic drug monitoring with a minimum detectable limit of 20 ng/ml. The method involves simple, one-step extraction procedure and analytical recovery was complete. The separation was performed on an analytical 150×4.6 mm i.d. μbondapak C₁₈ column. The wavelength was set at 235 nm. The mobile phase was 40% acetonitrile, 0.01 M sodium dodecyl sulphate, 0.01 M sodium dihydrogen phosphate, and distilled water to 100%, adjusted to pH 5.1 at a flow rate of 1.5 ml/min. The calibration curve was linear over the concentration range 0.2–2.5 μg/ml. The coefficients of variation for inter-day and intra-day assay were within the range of clinical usefulness.