Previously published data from this laboratory on the urinary excretion of 3-hydroxymethyl antipyrine (one of the major metabolites of antipyrine in man) have to be reconsidered, because of use of an impure reference substance in the high-pressure liquid chromatographic assay method. Comparison with newly synthesized, pure 3-hydroxymethyl antipyrine has shown that a correction factor of 0.45 needs to be applied to previously published data. Details of the synthesis of the pure reference 3-hydroxymethyl antipyrine are given.