Noradrenaline (NA) was assayed in brain regions of male and female WK and SHR of 6 to 40 weeks of age, using high performance liquid chromatography with electrochemical detection (HPLC-ECD). The change of levels with age was similar in brain regions of both strains except in brainstem and thoracic spinal cord where levels fell progressively with age in SHR but not in WK rats. The fall in NA content in these two regions is associated with a rise in blood pressure with age in the SHR and suggests a relationship between NA levels and hypertension. The demonstration of different patterns of change in NA levels with age between WK and SHR represents a new approach to resolving the problem of the lack of appropriate control animals for genetic models of disease. However, the data also emphasise that differences in catecholamine metabolism may occur between in-bred strains of rat such as the WK and SHR, and between male and female rats, unrelated to differences in blood pressure.