Nifedipine (NIF), a calcium channel antagonist, is metabolized primarily by cytochrome P450 (CYP3A4) to dehydronifedipine (DNIF). As such, NIF is often used as a probe drug for determining CYP3A4 activity in human studies. A rapid and sensitive liquid chromatography/tandem mass spectrometry (LC/MS/MS) method was developed and validated to simultaneously determine NIF and DNIF in human plasma using nitrendipine as the internal standard (IS). After extraction of the plasma samples by ether-n-hexane (3:1, v/v), NIF, DNIF and the IS were subjected to LC/MS/MS analysis using electro-spray ionization (ESI). Chromatographic separation was performed on a Hypersil BDS C(18) column (50 mm x 2.1 mm, i.d., 3 microm). The method had a chromatographic running time of approximately 2.5 min and linear calibration curves over the concentrations of 0.5-100 ng/mL for NIF and DNIF. The recoveries of the one-step liquid extraction method were 81.3-89.1% for NIF and 71.6-80.4% for DNIF. The lower limit of quantification (LLOQ) of the analytical method was 0.5 ng/mL for both analytes. The intra- and inter-day precision was less than 15% for all quality control samples at concentrations of 2, 10, and 50 ng/mL. The validated LC/MS/MS method has been successfully used to study pharmacokinetic interactions of NIF with the herbal antidepressant St. John's wort in healthy volunteers. These results indicated that the developed LC/MS/MS method was efficient with a significantly shorter running time (2.5 min) for NIF and DNIF compared to those methods previously reported in the literature. The presented LC/MS/MS method had acceptable accuracy, precision and sensitivity and was used in a clinical pharmacokinetic interaction study of NIF with St. John's wort, a known herbal inducer of CYP3A4. St. John's wort was shown to induce NIF metabolism with increased plasma concentrations of DNIF.