A high concentration of immunoreactive neuropeptide Y was observed in rat platelets using a specific and sensitive radioimmunoassay for neuropeptide Y. Three kinds of high performance liquid chromatography combined with radioimmunoassay for neuropeptide Y showed that immunoreactive neuropeptide Y in rat platelets is identical to rat authentic neuropeptide Y. To investigate the pathological role of platelet neuropeptide Y in genetic hypertensive rats, the platelet content and plasma concentration of neuropeptide Y were measured by a sensitive radioimmunoassay for rat neuropeptide Y in 5-, 10- and 15-wk old spontaneously hypertensive rat and age-matched Wistar Kyoto rat. Platelet content of immunoreactive neuropeptide Y in 5-, 10- and 15-wk old spontaneously hypertensive rat was higher than that in Wistar Kyoto rat at each age. No difference was observed in plasma concentration of immunoreactive neuropeptide Y between spontaneously hypertensive rat and Wistar Kyoto rat at each age.