A method was developed and validated for the analysis of R(−)-apomorphine, (R−)-apocodeine and R(−)-norapomorphine in human plasma and urine with N-propylnorapomorphine as internal standard using gas chromatography/mass spectrometry (GC/MS) and single-ion monitoring after a single liquid–liquid extraction and silylation of compounds. The quantification limits were 1 ng/ml for apomorphine and apocodeine and 25 ng/ml for norapomorphine. Calibration curves were linear, within the range 1–100 ng/ml. Variation in intraday and interday precision was below 10%. This method was applied to study apomorphine bioavailability in nine patients with Parkinson's disease before and after coadministration of a catechol-O-methyl transferase inhibitor. Copyright © 2005 John Wiley & Sons, Ltd.