An accurate and simple high-performance liquid chromatographic (HPLC) assay is presented for the quantitative determination of metaiodobenzylguanidine (MIBG) in human whole blood and plasma. The sample pretreatment involves a solid-phase extraction on Bakerbond SPE cyano columns. The HPLC system comprises a microBondapak C18 column and ammonium phosphate (pH4.0; 25 mM)-acetonitrile (80:20, v/v) as the mobile phase. Detection is performed by UV absorbance measurements at 254 nm. Linear regression with weighting factor l/chi 2 yielded the smallest sum of per cent relative concentration residuals over the concentration range of the assay (0.1-10 micrograms ml-1). MIBG levels, at the end of a 3-h infusion, in whole blood and plasma of carcinoid patients were measured and compared with the results obtained with radiodetection after addition of iodine-131-labelled MIBG.