A gas chromatographic-mass spectrometric assay is described for the determination of plasma nifedipine using nitrendipine as an internal standard. Chromatographic separation was performed on a dimethylsilicone capillary column, and detection was by selected ion monitoring of electron impact-generated base peak ions. The lower limit of quantifiable detection of nifedipine was 2 ng/ml. The method was applied to plasma samples obtained from a human subject who had been dosed with a 10-mg nifedipine capsule every 8 h for eight doses.