1. This study examined neuropeptide Y (NPY) concentrations in brain regions and peripheral tissues of young (3-4 months) and old (17-18 months) normotensive Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR). 2. Neuropeptide Y-like immunoreactivity (NPY-LI) was determined in kidney, adrenal, heart ventricles, atria and four brain regions, cerebral cortex, hypothalamus, ventrolateral medulla (VLM) and dorsomedial medulla containing the nucleus tractus solitarius (NTS), by radio-immunoassay following acid extraction. 3. Significant age-related increases in organ weights were observed in atria, ventricle and kidney of both WKY and SHR (P < 0.01). In order to take into account tissue hypertrophy, NPY-LI data were analysed as pmol/g tissue as well as total pmol/tissue. 4. At each age, similar NPY-LI concentrations were observed in WKY and SHR in all brain regions. A significant age-induced decrease in NPY-LI concentration and total NPY content was found in the hypothalamus of both WKY and SHR (P < 0.01). 5. In the cardiac ventricle, decreases were observed in NPY-LI concentration with ageing, and in SHR relative to WKY; however, no differences were observed in total NPY-LI content. A significant age-related increase in adrenal NPY-LI concentration was observed. No age- or strain-related alterations in atrial or renal NPY-LI were detected, with the exception of an increase in total kidney NPY-LI in WKY with ageing. 6. Thus in the periphery, few changes in NPY-LI were observed with genetic hypertension or with ageing.(ABSTRACT TRUNCATED AT 250 WORDS)