A highly sensitive and specific method was developed for the determination of histamine in biological materials by high-performance liquid chromatography with a cation exchanger and an automated Shore's fluorometric detection system. Since substances causing interference in Shore's o-phthalaldehyde method, such as ammonia, histidine, spermine and spermidine, were completely separated on the column and their fluorescent intensities were much less than that of histamine in this detection system, histamine could be determined by injecting a perchloric acid extract of human plasma or mouse brain tissue directly onto the column without any previous purification procedure. The lower limit of detection of histamine by this method is 0.05 pmol, and the within-day and day-to-day variations in plasma histamine assay are less than 3%. The plasma histamine level in normal human subjects was found to be 4.0 +/- 1.6 pmol/ml (mean +/- S.D., n = 20). A good linear correlation was obtained between values for the histamine contents of mouse brain tissues determined by this method and by a radioenzymatic method with a purified histamine-N-methyltransferase preparation. The histamine levels of whole brain, hypothalamus, thalamus, brain stem and frontal cortex in male ddY mice were 367 +/- 38, 1143 +/- 70, 414 +/- 66, 196 +/- 47 and 467 +/- 91 pmol/g of wet tissue (mean +/- S.D.), respectively.