A reversed-phase high performance liquid chromatographic method is presented by which the calcium channel blocker, nifedipine (NFP), may be measured in human serum using 17-alpha-ethinylestradiol as an internal standard. A mobile phase of phosphate buffer (0.01 M, pH 6.1)/methanol/acetonitrile (20:35:45) passed through a muBondapak C-18 column at 1.0 ml/min produced symmetrical bands for nifedipine and internal standard. A rapid and simple chloroform extraction of NFP from 1 ml serum samples proved to be efficient and reproducible. Recovery over a concentration range of 5-100 ng/ml was 92.3 +/- 5.1% (mean +/- SD, n = 6). Ultraviolet absorbance detection at 235 nm was sensitive to serum NFP concentrations of 5 ng/ml. Between-day coefficients of variation at 100 and 5 ng/ml were 4.0 and 11.4%, respectively (n = 10). Serum concentration data from NFP-treated subjects are presented.