A reversed-phase ion-pair high-performance liquid chromatographic procedure for quantitative determination of histamine and its metabolites in rat urine is described. This method allows simultaneous analysis of five major histamine metabolites. Good separations were obtained by using 1-pentanesulfonic acid as the ion-pair reagent. A gradient elution program was used; the total elution time was less than 22 min. Linear standard curves with high correlation coefficients were obtained. This procedure has the advantage of requiring little sample preparation time and handling, and therefore maximizes the recovery of metabolites.