Twenty-three amino acids and dipeptides, including compounds of neurochemical interest, are measured by high-performance liquid chromatography using electrochemical detection after precolumn derivatization with o-phthalaldehyde/beta-mercaptoethanol. The method uses a multistep polarity gradient system and the entire separation is performed in less than 23 min of analysis. The minimum detectable quantity was 0.66 pmol injected, corresponding to 50 nM concentration in the sample; the response was linear in the tested range of 1.33-1333 pmol (0.1-100 microM). Relative standard deviations ranged from 0.75 to 6.089% for area measurements (mean, 2.33) and from 0.209 to 0.779% for retention times (mean, 0.546). Examples of application of the method to analysis of biological samples such as rat brain homogenate and human cerebrospinal fluid are shown.