The nature of NPY-like immunoreactivity (NPY-LI) was investigated in plasma and tumour tissue of 1 7 pheochromocytoma patients by HPLC, gel filtration and isoelectric focusing using two radioimmunoassays (RIAs) directed against the C- and N-terminals of NPY respectively. The two RIAs gave similar results in pheochromocytomas: 86% of cases had higher NPY-LI concentrations than those found in normal adrenal glands and NPY-LI behaved like authentic human NPY during gel filtration and HPLC. Assessed by isoelectric focusing, NPY was found to be amidated in seven of nine tumours. Contrary to the findings obtained in tumours, the results of the two RIAs in plasma samples were not always concordant: compared to controls, elevated concentrations of NPY-LI were found in 86% of cases of pheochromocytomas using the C-terminally directed RIA and in 76% of cases using the N-terminally directed RIA. The results of HPLC and gel filtration of NPY-LI in plasma suggested that circulating Cand N-terminal NPY fragments account for the discrepancy between the results of the two RIAs. In conclusion, most pheochromocytomas contain large amounts of NPY-LI that behaved like authentic NPY by chromatographic analysis. On the contrary, circulating NPY-LI in some pheochromocytoma patients is heterogeneous with cleaved products which influence differently the power of the C- and N-terminally directed RIAs for the diagnosis of pheochromocytoma.