Phenethyl isothiocyanate is unstable in aqueous media and at low pH, and rapidly degrades to phenethylamine. Concentrations of phenethylamine, a phenethyl isothiocyanate marker, in dog plasma, were determined utilizing solid-phase extraction and gas chromatography-mass spectrometry with chemical ionization using acetone as the reagent gas. Deuterated d5-amphetamine was used as an internal standard. After extraction, phenethylamine and d5-amphetamine were derivatized using MBHFBA. Ions monitored for d5-amphetamine were m/z 337 and 338; and for phenethylamine were m/z 318 and 319. Precision and accuracy were studied using control solutions prepared in naive dog plasma (80 and 300 ng/ml). Intra-day variability was determined using six replicates of each control solution analyzed on a single day. The relative standard deviation for the 80 ng/ml control was 12.9% and for the 300 ng/ml it was 12.1%. Relative accuracy was 10.9% for the low control and -4.1% for the high control. Inter-day variability was determined over a 6-day period. For the 80 and 300 ng/ml control solutions, the relative standard deviations were 15.8 and 9.1%, respectively, and relative accuracy values were 10.1 and -5.2%, respectively. Standard curves were prepared in naive dog plasma and were linear over the range of phenethylamine assayed (10-500 ng/ml). The results of this study indicate that the proposed method is simple, precise, accurate and sensitive enough for analysis of large numbers of plasma samples.