An improved method for the determination of phentolamine in human plasma samples was developed. After being alkalinized, plasma samples (1 mL) were extracted with diethyl ether and then back-extracted with O.1 N HCl. Analyses were carried out on a Novapak C8 column eluted with a mixture of sodium monochloroacetate (pH 3) and acetonitrile (75:25). Amperometric detection was performed by oxidation at 1000 mV, using a glassy carbon electrode against Ag/AgCl. Calibration curves, constructed over a 1 to 30 ng/mL plasma concentration range, were linear (r=0.999). Intra-assay coefficients of variation and accuracy for the determined concentrations were comprised within 7.6–10.9% and 94.0–105.6%, respectively. Inter-assay coefficient of variation and accuracy ranges were 10.4–20.7% and 93.2–102.7%, respectively. The method's detection limit was 0.2 ng/mL, allowing determination of oral phentolamine pharmacokinetics after administration of a 40 mg dose.