An improved analytical assay was developed and validated for the quantification of milrinone concentrations in patients undergoing cardiac surgery. A solid-phase extraction was optimized to isolate milrinone from a plasma matrix followed by HPLC using UV detection. Plasma samples (1 ml) were extracted using a C(18) solid-phase cartridge. Milrinone was separated on a strong cation exchange analytical column maintained at 23.4 degrees C. The mobile phase consisted of a gradient (10:90 to 45:55), 0.05 M phosphate buffer (pH 3):acetonitrile. Calibration curves were linear in the concentration range of 1.25-320 ng/ml. Mean drug recovery and accuracy were respectively > or =96% and > or =92%. Intra- and inter-day precisions (CV%) were < or =6.7% and < or =7.9%, respectively. This method proved to be reliable, specific and accurate. Using different types of column for extraction and separation of milrinone proved to be necessary to achieve the sensitivity and specificity required when milrinone is given by inhalation.