A method is described for the determination of unconjugated terbutaline in plasma. The assay is based on gas chromatography chemical ionization mass spectrometry with ammonia as the reagent gas. Terbutaline and a deuterium labelled internal standard were isolated from plasma by ion exchange chromatography followed by extraction into t-butanol. Quantitation was performed by selected ion monitoring of the [MH]+ions m/z 442 and m/z 448 of the tri-O-TMS derivatives of terbutaline and the internal standard, respectively. Silylation was carried out in the presence of pyridine at room temperature and was complete within one minute. The overall recovery of terbutaline in the procedure was estimated at about 80%. Measurement of terbutaline down to 0.1 ng ml-1 in 4 ml of plasma is possible with a coefficient of variation of 8.6%. Day-to-day variation was found to be 6.0% at a level of 3.26 ng ml-1.