A high-pressure liquid chromatographic (HPLC) method was developed for the assay of antipyrine in small (0.1-ml) plasma samples using aminopyrine as the internal standard and a reversed-phase microparticulate column. The assay sensitivity (1 microgram/ml) permits development of a plasma level--time curve using a single rat. The mean (+/- SE) plasma elmination half-life in rats was 1.28 +/- 0.14 hr. A comparison of the spectrophotometric method with the HPLC method yielded a correlation coefficient of 0.98. The HPLC assay for antipyrine is rapid and precise and can be used for hepatic drug metabolism study in a single animal.