A simple, rapid, selective, accurate and precise method is described for the determination of risperidone and its active metabolite, 9-hydroxyrisperidone, in plasma using a chemical derivative of risperidone (methyl-risperidone) as the internal standard. The sample workup involved a single-step extraction of 1 mL plasma, buffered to pH 10, with heptane-isoamyl alcohol (98:2 v/v), then evaporation of the heptane phase and reconstitution of the residue in mobile phase. HPLC separation was carried out at on C18 column using a mobile phase of 0.05 m dipotassium hydrogen orthophosphate (containing 0.3% v/v triethylamine) adjusted to pH 3.7 with orthophosphoric acid (700 mL), and acetonitrile (300 mL). Flow rate was 0.6 mL/min and the detection wavelength was 280 nm. Retention times were 2.6, 3.7 and 5.8 min for 9-hydroxy risperidone, risperidone and the internal standard, respectively. Linearity in spiked plasma was demonstrated from 2 to 100 ng/mL for both risperidone and 9-hydroxyrisperidone (r = 0.999). Total imprecision was less than 13% (determined as co-efficient of variation) and the inaccuracy was less than 12% at spiked concentrations of 5 and 80 ng/mL. The limit of detection, determined as three times the baseline noise, was 1.5 ng/mL. Clinical application of the assay was demonstrated for analysis of post-dose (0.55-4.0 mg/day) samples from 28 paediatric patients (aged 6.9-17.9 years) who were taking risperidone orally for behavioural and emotional disorders.